Buffalo Shooting Suspect Was Held For Mental Health Evaluation Last Year

But Mr. Gendron picked his target carefully, the police said, choosing an area known for its large Black population and even visiting the neighborhood the day before the attack in what authorities described as “reconnaissance.”

And nearly a year before the mass shooting, his words had already caused alarm elsewhere.

The police said on Sunday that Mr. Gendron had been picked up at his high school last June by state police after making a threatening remark and had been taken to a hospital for a mental health evaluation.

Responding to a question for a class project about his post-graduation plans, Mr. Gendron said his involved a murder-suicide, a law enforcement official familiar with the case said.

But Mr. Gendron described the remark as a joke, the official said. And after the evaluation, which lasted about a day and a half, he was released, according to Joseph Gramaglia, the Buffalo police commissioner.

That account was confirmed by Special Agent Steven Belongia of the F.B.I., who said that Mr. Gendron was “not on the radar” of federal authorities.

Mr. Gendron, who the police said wore body armor and camouflage during his spree, is believed to have posted a lengthy screed riddled with racist writings and expressing admiration for a white supremacist ideology known as replacement theory, as well as for gunmen in other racist mass shootings.

“This individual came here with the express purpose of taking as many Black lives as he could,” said Mayor Byron Brown, a Democrat who is Buffalo’s first Black mayor.

The White House announced that President Biden and Jill Biden, the first lady, will visit Buffalo on Tuesday “to grieve with the community that lost 10 lives in a senseless and horrific mass shooting.” On Sunday night, the police identified the victims — a cross-section of a working-class neighborhood where the Tops store acted as both a crucial source of groceries and a community hub.

The dead included a retired Buffalo police officer, Aaron Salter Jr., 55, who worked at the grocery store as a security guard and was being hailed as a hero for confronting the gunman, and Ruth Whitfield, an 86-year-old grandmother of eight. Some died running errands: Celestine Chaney, 65, for example, who simply wanted to get strawberries to make shortcakes, or Roberta Drury, 32, who was just getting food for dinner. Heyward Patterson, 67, was killed helping to put groceries into another shopper’s car.

Four people were shot in the store’s parking lot and nine others inside, including Mr. Salter: He exchanged shots with the gunman, who was firing an assault weapon and protected by heavy body armor, according to Mark Poloncarz, the Erie County executive.

On Sunday, a patch of blood still stained the parking lot’s asphalt, as a range of state, federal and local officers worked the scene. The blocks surrounding the site were filled with elected officials and neighborhood mourners.

“A lot of my peers, my friends, the cop, they were in there,” said Karen Martin, 64, who came to the store on Sunday morning to pay her respects. “I just don’t believe that he did that.”

The sense of grief was also mixed with outrage. Local Black religious leaders pleaded with their white brethren in other parts of the state and country to do their part to counter racism and white supremacy.

“Don’t tell me you’re a friend of our community and you don’t address this today at your pulpit,” said Bishop Darius Pridgen of the True Bethel Baptist Church in Buffalo, adding, “If you do not stand behind those holy desks and acknowledge that there are still people who hate Black people, you can go to hell with the shooter for all I care. Because at the end of the day, if you’re silent right now, you are not a friend of mine.”

The attack on Saturday was the deadliest mass shooting in the United States this year, joining a grim roster of other racist massacres in recent years, including the killing of nine Black parishioners at a church in Charleston, S.C., in 2015; an antisemitic rampage in the Tree of Life synagogue in 2018 in Pittsburgh 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 and killed more than 20 people.

Extremists motivated by racial and ethnic hatred are considered the most dangerous threat among domestic terrorists. After a spate of horrific shootings targeting people of color and Jews in 2019, the F.B.I. elevated the threat to the highest level, meaning agents must prioritize developing confidential informants and take other steps to counter the violence.

Law enforcement officials said that Mr. Gendron, who has been charged with first-degree murder and pleaded not guilty on Saturday night, had traveled halfway across the state to commit his crime. The document he is believed to have written and posted online in the days leading up to the attack had mentioned that Buffalo was the nearest city to his home in the Southern Tier — a predominantly white region that runs along New York’s southern border with Pennsylvania — that had a major Black population.

On Sunday morning, F.B.I. agents and members of other law enforcement agencies gathered in front of Mr. Gendron’s home in Conklin, N.Y., a suburban town with rolling hills in the southern part of Broome County, about a 200-mile drive from Buffalo.

Neighbors there recalled watching Mr. Gendron play basketball in the driveway with his siblings, and some even had attended his front-yard high school graduation party last year, where they said there was no indication of trouble.

Others, however, said that there were signs of rebellion and odd behavior, including a moment after in-person schooling resumed when he wore a full hazmat suit to class.

“He wore the entire suit: boots, gloves, everything,” said Nathan Twitchell, 19, a former classmate at Susquehanna Valley High School.

Kolton Gardner, 18, of Conklin, who attended middle school and high school with Mr. Gendron, described him as “definitely a little bit of an outcast.”

“I knew he had an interest in guns, but where we grew up that wasn’t uncommon,” Mr. Gardner said.

That interest was apparently avid enough to encourage a purchase: Robert Donald, the owner of Vintage Firearms in Endicott, N.Y., said Sunday that he recently sold a Bushmaster assault weapon to Mr. Gendron.

“I just can’t believe it. I don’t understand why an 18-year-old would even do this,” said Mr. Donald, 75, who primarily sells collectible firearms. “I know I didn’t do anything wrong, but I feel terrible about it.”

Mr. Gendron’s writings were littered with racist, anti-immigrant views that claimed white Americans were at risk of being replaced by immigrants or people of color, once-fringe ideas that have been given a fuller airing in recent years by some prominent conservative commentators.

On the far right, the theory, which sometimes blames Jews for fomenting the “great replacement,” has been tied to gunmen in several other mass shootings as well as the 2017 right-wing rally in Charlottesville, Va. that devolved into violence.

At a midday news conference on Sunday, Mr. Gramaglia, the police commissioner, said that state and federal authorities had sought warrants for information about Mr. Gendron’s digital activities. They pursued access to his computers and phones, as well as searches of his home and vehicle. He added that authorities believed that Mr. Gendron acted alone.

He was placed on suicide watch and separated from the general population at the Erie County Holding Center, said John Garcia, the Erie County sheriff, who refused to say the suspect’s name — referring to him by his inmate identification number — and called his actions “pure evil.”

Mr. Gendron surrendered after putting his weapon to his chin, said Mr. Gramaglia, who praised his officers for their fast response to the shooting. Still, some community members wondered how Mr. Gendron — who authorities said had two other guns in the car he drove to the massacre — had not been shot by police during his attack, something they said would have happened had he been Black.

On Sunday, however, Mr. Gramaglia rebutted this suggestion, saying that his officers always worked to de-escalate violent situations. “We’re not looking to shoot anyone,” he said, noting that Mr. Gendron had pointed the gun at himself, not police.

Mr. Gendron live-streamed his attack, the police said, capturing the images of chaos he caused with a camera affixed to his helmet. The video was broadcast on Twitch, a livestreaming site owned by Amazon that is popular with gamers, though the site took the channel offline almost immediately after the attack started. Still, images of the broadcast could still be found online; a snippet of the video of the shooting was viewed more than three million times on a site called Streamable before it was removed.

At a morning appearance at the True Bethel church, Ms. Hochul, a Democrat who is a Buffalo native, said she was angry at the violence that had shaken her hometown, calling the gunman “a coward.” But she also expressed deep frustration with “the social media platforms that allow this hatred to ferment and spread like a virus.”

When pressed on how she planned to confront such hate speech online, without impinging on First Amendment rights, Ms. Hochul noted that “hate speech is not protected” and said she would soon be calling meetings with social media companies.

“I assure you when I get back to Albany, their phones will ring,” she said.

Along with other Buffalo residents, Ms. Hochul stressed that she wanted the city to be known as a turning point in the nation’s string of gun tragedies.

“I want them to talk about Buffalo,” she said, “as the last place this ever happened.”

Christine Chung, Andy Newman, Ashley Southall, Ali Watkins, Emma Bubola, Chelsia Rose Marcius, Troy Closson, Mihir Zaveri, Dan Higgins, Chris Cameron, Kellen Browning, Nicholas Confessore, Grace Ashford and Adam Goldman contributed reporting.