The security guard who was fatally shot confronting the gunman has been identified.

Shortly after Mr. Gendron was captured, a manifesto believed to have been posted online by the gunman emerged, riddled with racist, anti-immigrant views that claimed white Americans were at risk of being replaced by people of color. In the video that appeared to have been captured by the camera affixed to his helmet, an anti-Black racial slur can be seen on the barrel of his weapon.

The attack, at a Tops Friendly Market in a largely Black neighborhood in east Buffalo, conjured grim comparisons to a series of other massacres motivated by racism, including the killing of nine Black parishioners at a church in Charleston, S.C., in 2015; an antisemitic rampage in a Pittsburgh synagogue in 2018 that left 11 people dead; and an attack at a Walmart in El Paso in 2019, where the man charged had expressed hatred of Latinos. More than 20 people died there.

In the Buffalo grocery store, where four employees were shot, the savagery and planning were evident: Mr. Gendron was armed with an assault weapon and wore body armor, the police said. And his preferred victims seemed clear as well: All told, 11 of the people shot were Black and two were white, the authorities said.

“It was a straight up racially motivated hate crime,” John Garcia, the Erie County sheriff, said.

And House Speaker Nancy Pelosi said that the views the gunman expressed in his manifesto were part of a right-wing “philosophy in our country.” She said in a later statement that the House would take up legislation that would “strengthen efforts to combat domestic terrorism.”

“That’s what this is, domestic terrorism,” Ms. Pelosi said on CNN’s “State of the Union,” separately calling for passage of federal legislation to expand gun background checks, which she said was a “huge priority” for Democrats.

Senator Bernie Sanders of Vermont, a democratic socialist, also called the attack an “act of domestic terrorism by a racist, antisemitic white supremacist,” and called for the passage of “common-sense gun safety reforms.”

Mr. Biden, arriving at Joint Base Andrews in Maryland on Sunday morning, said he had not yet spoken to the families of victims. He had said earlier that he was not sure if he would be able to visit Buffalo before a planned trip to Asia this week.

In a statement, Vice President Kamala Harris expressed her condolences to the victims and their families, echoing calls from President Biden to clearly condemn the shooting as an attack motivated by racism and hate.

“What is clear is that we are seeing an epidemic of hate across our country that has been evidenced by acts of violence and intolerance,” she said. “We must call it out and condemn it. Racially-motivated hate crimes or acts of violent extremism are harms against all of us.”

Senator Chris Murphy, Democrat of Connecticut and a champion of gun safety legislation, said on MSNBC that “in the wake of this Buffalo shooting, it may be that we have to put a vote up in the Senate or in the House — show the American people where folks stand.”

Legislation to expand background checks and impose other safeguards for purchasing a gun has stalled in the evenly divided Senate, where 60 votes are needed to advance most legislation.

Transportation Secretary Pete Buttigieg said on Sunday that the shooting was an act of terrorism, but did not say whether the Biden administration would support a new federal law criminalizing domestic terrorism.

“We don’t know, obviously, all of the details that fit the legal definitions,” Mr. Buttigieg said on CBS’s “Face the Nation.”

“What we know is somebody traveled a long distance with an AR-15 to hunt human beings, to hunt Black people. We need to make sure we root out that kind of hate.”

Mr. Salter and the gunman exchanged fire, but the gunman was wearing heavy body armor, and the bullet bounced off, Mark Poloncarz, the Erie County executive, said at a vigil near the store on Sunday, when he identified Mr. Salter as the guard. The gunman fatally shot Mr. Salter and went on to kill six more people in the store.

“He’s a true hero, and we don’t know what he prevented. There could have been more victims if not for his actions,” Joseph Gramaglia, the police commissioner in Buffalo, said of Mr. Salter in an appearance on ABC’s “This Week.”

Mr. Salter, 55, had spent decades as an officer in the Buffalo Police Department before retiring and taking a job as a security guard with the Tops franchise, according to The Buffalo News. During his career in the department, Mr. Salter was featured several times in local news reports.

According to reporting in The Buffalo News, he and a partner once chased down an arsonist. In a separate instance described by the paper, Mr. Salter once faced down a burglary suspect who was armed with a shotgun.

Mr. Poloncarz said the families of all 10 victims have been notified, and their names are expected to be officially released tomorrow.

“I knew nothing about it until I got the call from them. I couldn’t believe it,” he said.

“I just can’t believe it. I don’t understand why an 18-year-old would even do this,” he added. “I know I didn’t do anything wrong, but I feel terrible about it.”

Two agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives then showed up to his store at about 7:30 p.m. on Saturday to collect paperwork from Mr. Donald, including purchase-history documents detailing when Mr. Gendron bought the firearm.

Mr. Donald said he did not remember Mr. Gendron among the many customers he gets throughout the year, even though he sells very few assault weapons a year — around six at the most, he said.

Mr. Donald said he did a background check on Mr. Gendron before he sold him the gun. The report showed nothing.

“He didn’t stand out — because if he did, I would’ve never sold him the gun,” Mr. Donald said.

Mr. Gendron identified the firearm in his manifesto as a Bushmaster XM-15. The powerful assault weapon can inflict heavy damage. An official briefed on the investigation said positive identification of many of the victims has been delayed by the severity of their wounds.

Mr. Gendron wrote that he used his father’s power drill to modify the gun, using a parts kit that retails for $59.99 online. Mr. Donald said that when he sold Mr. Gendron the firearm it had complied with New York laws.

“Even with all of those safety features on it — which is the only way I sell it — any gun can be easily modified if you really want to do it,” he said.

Dan Higgins contributed reporting.

“Wild access to guns unfettered — we need national laws to deal with this,” Ms. Hochul said.

She added that tech companies should re-evaluate their platforms to restrain dangerous content.

“These theories that result in the radicalization of a young person, sitting in their house, is deeply scary,” Ms. Hochul said. “And it’s something that has to be dealt with.”

Speaking to parishioners later Sunday at True Bethel Baptist Church in Buffalo, Ms. Hochul expressed anger at the accused gunman — calling him a “coward” — and vowed to press for changes to prevent future mass shootings in New York.

“We must do something about this; we will not just call it out,” said Ms. Hochul, a Buffalo native.

“I want them to talk about Buffalo as the last place this ever happened,” she said, adding, “Our white brothers and sisters need to be standing up as well in churches all across this state, all across this nation. Because an attack on one of us is an attack on all of us.”

Byron Brown, the mayor of Buffalo, also called for stricter gun control on Sunday, as well as stricter controls of hate speech “that has been allowed to proliferate on social media and the internet.”

“We have to put more pressure on lawmakers in Washington,” Mr. Brown said in an interview on “Face the Nation” on CBS. “Those that have been obstructionists to sensible gun control, to reforming the way guns are allowed to proliferate and fall into the wrong hands in this country.”

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Segregation is also a root cause, according to experts, of why efforts to bring an economic renaissance to Buffalo have done little for Black residents. A University of Buffalo report in 2021 found that living conditions for Black residents of the city, across measures of health, housing, income and education, had improved little and in some cases had declined over the preceding 30 years.

India B. Walton, a nurse and community activist who nearly unseated Buffalo’s four-term Democratic mayor last year, called the city “segregated by design” as she reacted to the shooting on Twitter.

“Our government can’t prevent things that they actually cause,” she wrote.

Indeed, like many major American cities, Buffalo bears a tell-tale scar of long-standing segregation, a highway built in the 1950s and 1960s that cut directly through a Black neighborhood, severing those communities and stifling economic development for decades to come.

That highway, the Kensington Expressway, is about two blocks from the Tops supermarket where at least 10 people were shot and killed on Saturday. (Also near the scene of the shooting is City Honors High School, a magnet high school regularly ranked as one of the best in the country that has struggled to improve its disproportionately low Black enrollment.)

The destruction caused by the creation of the Kensington Expressway included the razing of Humboldt Parkway, a tree-lined public space designed by the legendary architect Frederick Law Olmsted. It has been described as the “spine” of the Black middle-class neighborhood that was emerging at the time.

Pollution from the expressway, which helped give residents of mostly white suburbs easy access to the city center, has done long-term damage to the health of the people living near it. The road also cut Black residents of the East Side off from key community institutions like banks and grocery stores, according to a 2018 report from the Partnership for the Public Good.

The very existence of the Tops grocery store was a hard-won victory for East Siders. The neighborhood is a food desert, and residents had been campaigning for a store like Tops for more than a decade when it finally opened in 2003, according to The Buffalo News.

The dearth of options helped make Tops a neighborhood establishment. Many families were shopping for their Sunday dinners there when the shooting began. The suspected gunman said in his manifesto that, in addition to targeting the area for its Black population, he had researched the days and times when Tops was busiest.

The Buffalo Community Fridge network, a mutual aid group that stocks community refrigerators with fresh produce and prepared meals for neighbors — including one that is less than half a mile from Tops — said it received many donations on Saturday, after its call to support residents affected by the shooting was shared on social media.

The group is organizing volunteers to buy and distribute food in the area on Sunday.

William K. Rashbaum and Alexandra E. Petri contributed reporting.

His manifesto, which he posted online shortly before the killings, contained old strains of white supremacy and xenophobia. But his rampage, in what was then a grim new twist, was packaged for a live audience online.

In the three years since, the Christchurch massacre has served as a grotesque touchstone for racist violence around the world, with some, including the man suspected of killing 10 people in the attack on a grocery store in Buffalo on Saturday, appearing to draw inspiration from it.

Payton Gendron, the white man whom the police accused of shooting 13 people at the store in a largely Black neighborhood in Buffalo, described the New Zealand gunman, Brenton Tarrant, as the person who had the biggest influence on his radicalization. “Brenton’s livestream started everything you see here,” he wrote.

Like the Christchurch killer, the Buffalo gunman scrawled a racist slur on his weapon and broadcast the attack on a livestreaming site. He wrote out a lengthy manifesto that was steeped in the “great replacement” doctrine, which argues that whites are at risk of being replaced by people of color.

The Christchurch massacre has also been cited by the perpetrators of other attacks.

• Prosecutors said a man who killed one person and injured three in an attack on a synagogue near San Diego during the Jewish holiday of Passover in April 2019 was inspired by the shootings in New Zealand. The gunman was sentenced to life without the possibility of parole last year.

• A gunman who killed 23 people in a crowded shopping center in El Paso, Texas, in August 2019 wrote a 2,300-word screed, posted online minutes before the attack, that warned of a “Hispanic invasion of Texas.”

  “In general, I support the Christchurch shooter and his manifesto,” it said. “This attack is a response to the Hispanic invasion of Texas.”

• A 22-year-old Norwegian man who was sentenced to 21 years in prison for killing his stepsister and opening fire on a mosque said during his trial that he was inspired by the New Zealand massacre.

Three of his victims outside were killed, the police said later. One was wounded.

Then he entered the busy grocery store and continued his rampage, broadcasting the footage live online.

A retired Buffalo police officer who was working as a security guard fired multiple shots at the gunman. The authorities said that the suspect was struck but was wearing body armor and managed to return fire, killing the security guard.

He shot shoppers and employees, according to the police, leaving a trail of bodies in the aisles. Screenshots of the broadcast circulating online appeared to show the gunman, at one point, holding a gun and standing over a body.

Shonnell Harris, an operation manager at the Tops, told The Buffalo News that she was stocking shelves when she heard loud noises and saw people running toward the back of the store. Ms. Harris stumbled several times as she ran for her life, she said, before managing to escape through the rear door. She estimated that she heard more than 70 shots.

And she saw the gunman. “He had army fatigue stuff on,” she told The Buffalo News.

Eventually, the gunman returned to the front of the store, where he was confronted by Buffalo police in the vestibule. He then put the gun to his own neck, said the police commissioner, Joseph A. Gramaglia, at a news conference.

Two patrol officers on the scene persuaded the gunman to drop the gun, Mr. Gramaglia said. He began removing some of his tactical gear in surrender, according to officials. Then the police tackled him, witnesses told The Buffalo News.

In the end, 10 people were dead, with three sustaining what appeared to be non-life threatening injuries. Of the 13 people shot, 11 were Black.

At some point during the shooting, Dominique Calhoun pulled into the parking lot, about to treat her two daughters to ice cream, when she suddenly saw people running out of the store screaming. “That literally could’ve been me,” she said.

“That literally could’ve been me,” Ms. Calhoun said. “I’m just in shock. I’ve never had something like this happen so close to home.”

Ken Stephens, 68, a member of a local anti-violence group, described a grisly scene. “I came up here, and bodies were everywhere,” he said.

News of the shooting spread quickly across the city. Marilyn Hanson, 60, raced to Tops to make sure her daughter, who lived nearby, wasn’t among the victims; she was safe.

Both Ms. Hanson and her daughter shop at the store often.

“My daughter was so scared because that could’ve been me in that store,” Ms. Hanson said, adding: “If a Black man did this, he’d be dead, too,” referring to the fact that the gunman had surrendered and been taken into custody.

Daniel Love, 24, was inside his Love Barber Shop near the supermarket with his wife when he heard a noise, he said. His wife is from Iraq and immediately recognized the sound of gunfire. He told her to get down, he said. He eventually ran to the parking lot and saw the lifeless body of someone he knew.

Ulysees O. Wingo Sr., a member of the Buffalo Common Council who represents a district adjacent to the site of the shooting, said he also knew some victims. As he spoke, onlookers gathered at the site, with about 100 standing along a side street. Yellow police tape cordoned off the block surrounding the store, and at least two dozen police officers, along with several vehicles, guarded the perimeter.

“This is the largest mass shooting to date in the city of Buffalo,” Mr. Wingo said. “I don’t think anyone here in the city of Buffalo thought that something like this could ever happen, would ever happen.”

Mr. Wingo said most of the shoppers at the Tops supermarket were Black, mirroring the surrounding neighborhood.

Dorothy Simmons, 64, typically spends part of her Saturdays at Tops, shopping for food to prepare for Sunday dinner. “That’s what we do in this community,” said Ms. Simmons, who has lived in East Buffalo all her life. On this Saturday, Ms. Simmons was at work in Amherst when she heard the news. She cried, she said. “This is our store — this is our store,” Ms. Simmons said.

Ms. Simmons, who is Black, said the fact that the gunman was able to surrender showed disparity.

“If that had been my son, it would have never been surrender. We never had a chance to surrender,” Ms. Simmons said. “It would never be that way.”

Dan Higgins contributed reporting from Buffalo, New York.

And it has repeatedly been the motivation for attacks across the United States and beyond, from the Poway, Ca. synagogue shooting in 2019 to the killing of 51 worshipers at two mosques in Christchurch, New Zealand, the same year.

The racist theory was directly referenced in a four-page screed written by the man charged with killing more than 20 people in El Paso, which described an attack in response to “the Hispanic invasion of Texas” and outlined fears about the group gaining power in the United States.

One year earlier, when 11 people were killed at Tree of Life synagogue in Pittsburgh, the accused gunman had espoused similar racist views, referring to the people helped by a Jewish agency that aids refugees as “invaders.”

The theory was conceived in the early 2010s by Renaud Camus, a French author who has written about fears of a white genocide, arguing that immigrants who give birth to more children represent a threat to white people.

Mr. Camus has attempted to distance himself from violent white supremacists, decrying killings even as his ideas have been referenced in more attacks. But he told The New York Times in 2019 that he still stands by the notion.

The idea that white people should fear being replaced by “others” has spread through far-right online platforms, shaping discussions among American white nationalists, The Times has reported.

It has also been evident across some acts of violence. About 60 percent of the extremist murders committed in the United States between 2009 and 2019 were committed by people espousing white supremacist ideologies like the replacement theory, the Anti-Defamation League found.

“It is the most mass-violence-inspiring idea in white supremacist circles right now,” said Heidi Beirich, co-founder of the Global Project Against Hate and Extremism. “This particular idea has superseded almost everything else in white supremacist circles to become the unifying idea across borders.”

Experts have said the belief represents a shift in the conversations of white supremacists. Several decades ago, they often proclaimed that they were superior because of their race. While that continues today, many now focus on the idea that they fear extinction at the hands of people of color. At a racist rally in Charlottesville, Va. in 2017, marchers chanted, “Jews will not replace us.”

Mr. Gendron, an 18-year-old white man, espoused similar views in the manifesto, directly referencing “racial replacement” and “white genocide.” The first page contained a symbol known as the sonnenrad, or black sun — two concentric circles with jagged beams emanating from the center. The Anti-Defamation League has said it was commonly used in Nazi Germany, and has now been adopted by white supremacists and neo-Nazis.

Mr. Gendron praised nationalism and blamed European men for allowing themselves to get “ethnically replaced.” He lamented diversity in America, writing that people of color should “leave while you still can.” And he criticized progressives, saying they had succeeded only at “teaching white children to hate themselves.”

Ms. Beirich, who reviewed the manifesto on Saturday, said it seemed to contain a “hodgepodge of every crazy white supremacist idea.”

It unfolded in a largely Black neighborhood in Buffalo, and 11 of the people shot were Black, officials said. Mr. Gendron wrote in his manifesto that he had selected the area because it held the largest percentage of Black residents near his home in the state’s Southern Tier.

On Saturday evening, authorities pored over the document, which outlined each step of a plan to kill as many Black people as possible.

He named the Bushmaster semiautomatic assault rifle he would use. He constructed a full timeline of the day, detailing the parking spot he would drive to, where he would eat beforehand and where he would livestream the violence. And he had carefully studied the layout of the grocery store, writing that he would shoot a security guard near the entrance before walking through aisles and firing upon Black shoppers, shooting them twice in the chest when he could.

His writings were also riddled with racist, anti-immigrant views arguing that white Americans are at risk of being replaced by people of color, a common trope on the far-right known as the “great replacement” theory. The same ideas have motivated gunmen in several other mass shootings.

Mr. Gendron wrote that he was inspired by the perpetrators of other white supremacist acts of violence, naming Dylann Roof, who killed nine Black parishioners in South Carolina in 2015, among other gunmen. His plan for the shooting in Buffalo resembled the 2019 massacre at a Walmart in El Paso, Tex., in which more than 20 people died and the gunman had also posted a four-page screed filled with white supremacist views.

He said that he felt a particular connection to Brenton Harrison Tarrant — calling him the person “who had radicalized him the most.” Mr. Tarrant was sentenced to life without parole for killing 51 Muslims during Friday prayer at two mosques in Christchurch, New Zealand. Mr. Gendron said that he had watched Mr. Tarrant’s livestream of the attack and read his writings.

Buffalo officials said that Mr. Gendron had “traveled hours from outside” the neighborhood to unleash gunfire at unsuspecting shoppers at an outlet of the regional grocery chain Tops Friendly Markets. He lived in the Southern Tier with his parents and two brothers, according to the manifesto.

A spokeswoman at SUNY Broome Community College near Binghamton added that he was a former student whose dates of attendance were not immediately known.

Mr. Gendron’s writings depicted a man who grew to hold racist views in recent years as he visited fringe online spaces. His beliefs and ideology had moved farther right over the past three years, he wrote.

Around May 2020, during a period of pandemic boredom, Mr. Gendron said that he had begun to frequent 4chan, an anonymous forum, including its Politically Incorrect message board. There, he said, he was exposed to the conspiracy theory that white people are at risk of being replaced.

He had been “passively preparing” for the attack in Buffalo for several years, purchasing ammunition and gear, while infrequently practicing shooting, he wrote. Around January, he wrote, the plans “actually got serious.”

Eduardo Medina and Vimal Patel contributed reporting.