US federal agents have questioned the parents of the 18-year-old white man accused of shooting and killing 10 people in a New York supermarket, according to a law enforcement official.

Federal authorities were still working Sunday to confirm the authenticity of a 180-page manifesto posted online that detailed the plot and identified Payton Gendron by name as the shooter, the official said.

Authorities say the shooting was motivated by racial hatred.

The official, who was not authorized to discuss details of the investigation and spoke on condition of anonymity, said Mr. Gendron’s parents were cooperating with investigators.

A preliminary investigation found Mr. Gendron repeatedly visited sites espousing white supremacist ideologies and race-based conspiracy theories and conducted extensive research into the 2019 mosque shooting in Christchurch, New York. Zealand, and about the man who had killed dozens of people at a summer camp in Norway in 2011, the official mentioned.

Mr. Gendron lives about 320 kilometers from the Buffalo grocery store where the shooting took place.

The market is located in a predominantly African-American neighborhood.

“It’s too much. I’m trying to testify, but it’s too much,” Buffalo resident Yvonne Woodard said.

On Sunday, Buffalo Police Commissioner Joseph Gramaglia said Gendron was in town “at least the day before.”

Police allege Gendron shot and killed 11 blacks and two whites on Saturday during a rampage he broadcast live before turning himself in to authorities.

Screenshots believed to be from the Twitch broadcast appear to show a racial epithet scrawled on the rifle used in the attack, along with the number 14, a likely reference to a white supremacist slogan.

“Domestic terrorism, pure and simple”

A vigil was held outside the Buffalo supermarket following the attack. And on Sunday morning, state Attorney General Letitia James spoke at a moving church service.

“It was domestic terrorism, plain and simple.”

Among the dead was security guard Aaron Salter, a retired Buffalo police officer, who fired multiple shots at the killer, Gramaglia said Saturday.

A bullet hit the shooter’s armor, but had no effect. The shooter then killed Mr. Salter, before causing other victims.

“He cared about the community. He took care of the store,” Yvette Mack, who had shopped at Tops earlier on Saturday, said of Mr Salter.

“He did a good job, you know. He was very nice and respectable.”

Ruth Whitfield, 86, mother of retired Buffalo fire marshal Garnell Whitfield, was also killed.

Buffalo Mayor Byron Brown told worshipers he saw the former firefighter at the scene of the shooting on Saturday, looking for his mother.

“My mother had just visited my father, as she does every day, in the nursing home and stopped at Tops to buy some groceries. And no one heard from her” , Mr. Whitfield told the mayor at the time.

She was confirmed as a victim later that day, Mr Brown said.

Katherine Massey, who had gone to the store to shop, was also killed, according to the Buffalo News.

The names of the other victims have not been released.

Twitch said in a statement that it ended the broadcast of the shooter “less than two minutes after the violence began.”

The mass shooting further unsettled the United States, a nation wracked by racial tensions, gun violence and a string of hate crimes.

A day before the incident, Dallas police said they were investigating shootings in the city’s Koreatown as hate crimes.

The Buffalo attack came just a month after a Brooklyn subway shooting left 10 people injured, and just over a year after 10 people were killed in a Colorado supermarket shooting.

Mr. Gendron was arrested on Saturday for murder.

ABC/PA