BUFFALO — The white 18-year-old who fatally shot 10 people at a Buffalo supermarket researched the local demographics and arrived a day in advance to conduct reconnaissance with the intent of killing as many Black people as possible, officials said Sunday.

The racially motivated attack came a year after the gunman was taken to a hospital by State Police after making threats involving his high school, according to authorities.

He wasn’t charged with a crime and was out of the hospital within a day and a half, police said, but the revelation raised questions about his access to weapons and whether he could have been under closer supervision by law enforcement.

The Buffalo attack prompted grief and anger in the predominantly Black neighborhood around Tops Friendly Market. A group of people gathered there Sunday afternoon to lead chants of “Black lives matter” and mourn victims that included an 86-year-old woman who had just visited her husband in a nursing home and a supermarket security guard, both of whom were Black.

“Somebody filled his heart so full of hate that he would destroy and devastate our community,” the Rev. Denise Walden-Glenn said.

Speaking at the National Peace Officers’ Memorial service at the U.S. Capitol, President Joe Biden said, “We must all work together to address the hate that remains a stain on the soul of America.” The White House later announced that the president and first lady would travel to Buffalo on Tuesday to “grieve with the community.”

The Buffalo attack was the deadliest of multiple shootings across the country in recent days. Officials in Milwaukee imposed a curfew after 21 people were injured in three separate shootings near an entertainment district where thousands gathered Friday for an NBA playoff game. Three other shootings over the weekend in the Midwest city left three people dead.

On Sunday, two shootings — one at a Houston flea market and another at a California church — left three people dead and others wounded.

As the country reeled from the Buffalo attack, new details emerged about the gunman’s past and Saturday’s rampage, which the shooter livestreamed on Twitch. New York Gov. Kathy Hochul, a Buffalo native, demanded technology companies tell her whether they’ve done “everything humanly possible” to make sure they’re monitoring violent content as soon as it appears.

“If not, then I’m going to hold you responsible,” she said.

Twitch said in a statement that it ended the transmission “less than two minutes after the violence started.”

New York State Police said troopers were called early last June to the high school then attended by the alleged gunman, Payton Gendron, for a report that a 17-year-old student had made threatening statements.

Gendron threatened to carry out a shooting at Susquehanna Valley High School, in Conklin, New York, around the time of graduation, a law enforcement official who spoke on condition of anonymity said. The official was not authorized to speak publicly on the investigation.

Buffalo Police Commissioner Joseph Gramaglia said Gendron had no further contact with law enforcement after his release from the hospital.

“Nobody called in,” he said. “Nobody called any complaints,” Gramaglia said.

Federal law bars people from owning a gun if a judge has determined they have a “mental defect” or they have been forced into a mental institution — but an evaluation alone would not trigger the prohibition.

Federal authorities were still working to confirm the authenticity of a racist 180-page document, purportedly written by Gendron, that detailed his plans for the attack and reasons for carrying it out.

A preliminary investigation found Gendron had repeatedly visited sites espousing white supremacist ideologies and race-based conspiracy theories and extensively researched the 2019 mosque shootings in Christchurch, New Zealand, and the man who killed dozens at a summer camp in Norway in 2011, the law enforcement official told AP.

Federal agents served multiple search warrants and interviewed Gendron’s parents, who were cooperating with investigators, the law enforcement official said.

Portions of the Twitch video circulating online showed the gunman firing volley after volley of shots in less than a minute as he raced through the parking lot and then the store, pausing for just a moment to reload. At one point, he trains his weapon on a white person cowering behind a checkout counter, but says “Sorry!” and doesn’t shoot.

Screenshots purporting to be from the broadcast appear to show a racial slur targeting Black people scrawled on his rifle, as well as the number 14 — likely referencing a white supremacist slogan.

Authorities said he shot, in total, 11 Black people and two white people Saturday.

“This individual came here with the express purpose of taking as many Black lives as he possibly could,” Buffalo Mayor Byron Brown said at a news conference Sunday.

The lengthy statement attributed to Gendron outlined a racist ideology rooted in a belief that the United States should belong only to white people. All others, the document said, were “replacers” who should be eliminated by force or terror. The attack was intended to intimidate all non-white, non-Christian people and get them to leave the country, it said.

The document said Gendron researched demographics to select his target, and picked a neighborhood in Buffalo because it had a high ratio of Black residents.

Gendron traveled about 200 miles (320 kilometers) from his home in Conklin, New York, to Buffalo to commit the attack, police said.

He conducted reconnaissance on the store and the area on Friday, a day before the shooting, Gramaglia said.

Gendron surrendered to police who confronted him in the supermarket’s vestibule and convinced him to drop the rifle he had put to his neck. He was arraigned later Saturday on a murder charge, appearing before a judge in a paper gown.

The Buffalo attack was just the latest act of mass violence in a country unsettled by racial tensions, gun violence and a recent spate of hate crimes. It came a month after a shooting on a Brooklyn subway wounded 10, and just over a year after 10 were killed in a shooting at a Colorado supermarket.

“It’s just too much. I’m trying to bear witness but it’s just too much. You can’t even go to the damn store in peace,” Buffalo resident Yvonne Woodard told the AP. “It’s just crazy.”