The FBI is interviewing the parents of Payton Gendron, the white 18-year-old who allegedly shot and killed 10 people at a Buffalo supermarket in what was reportedly a racially-motivated attack. 

The US teen researched the local demographics of the area and arrived a day in advance to conduct reconnaissance with the intent of killing as many Black people as possible, law-enforcement sources have said. 

Payton Gendron appears during his arraignment in Buffalo City Court. Photo / AP 

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

Gendron 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. 

Parents of the teen, Paul and Pamela Gendron – both civil engineers with the state Department of Transportation – are co-operating with investigators, sources told AP. 

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

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

Speaking at the National Peace Officers' Memorial service at the US Capitol, President Joe Biden said, "We must all work together to address the hate that remains a stain on the soul of America." 

Payton Gendron, left, with his parents Paul and Pamela and siblings. Photo / via Facebook 

Gendron, right, has pleaded not guilty to first-degree murder charges. Photo / via Facebook 

As the country reeled from its latest mass shooting, new details emerged about the gunman's past and Saturday's rampage, which the shooter livestreamed on Twitch. 

New York Governor 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." 

Gendron travelled about 320km from his home in Conklin, New York, to Buffalo to commit the attack, police said. Investigators believe Gendron specifically researched the demographics of the population around Tops Friendly Market, the official said. 

A Buffalo police officer talks to children at the scene of Saturday's shooting at a supermarket. Photo / AP 

A cyclist pauses passes the scene of the shooting at the supermarket, in Buffalo. Photo / AP 

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 on Saturday on a murder charge, appearing before a judge in a paper gown. 

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

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

People march to the scene of the shooting at the supermarket, in Buffalo. Photo / AP 

People embrace outside the scene of the shooting at a supermarket. Photo / AP 

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 authorised 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. 

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 on 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 on 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 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." 

- additional reporting news.com.au