By Carolyn Thompson And Michael Balsamo in Buffalo

BUFFALO, N.Y. (AP) — Federal agents interviewed the parents of the teenager accused of shooting and killing 10 people at a Buffalo supermarket and served multiple search warrants, a law enforcement official told The Associated Press on Sunday.

Federal authorities were still working to confirm the authenticity of a 180-page manifesto that was posted online, which detailed the plot and identified Gendron by name as the gunman, the official said. Authorities say the shooting was motivated by racial hatred.

Payton Gendron’s parents were cooperating with investigators, the official said. The official was not authorized to discuss details of the investigation into the Saturday afternoon shooting publicly and spoke to AP on condition of anonymity.

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 official said.

It wasn’t immediately clear why Gendron had traveled about 200 miles from his Conklin, New York, to Buffalo and that particular grocery store, but investigators believe Gendron had specifically researched the demographics of the population around the Tops Friendly Market and had been searching for communities with a high number of African American residents, the official said. The market is located in a predominantly Black neighborhood.

“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.”

In a Sunday interview with ABC, Buffalo Police Commissioner Joseph Gramaglia said that Gendron had been in town “at least the day before.”

“It seems that he had come here to scope out the area, to do a little reconnaissance work on the area before he carried out his just evil, sickening act,” Gramaglia said.

Police said Gendron shot, in total, 11 Black people and two white people Saturday in a rampage that the 18-year-old broadcast live before surrendering to authorities. Screenshots purporting to be from the Twitch broadcast appear to show a racial epithet scrawled on the rifle used in the attack, as well as the number 14, a likely reference to a white supremacist slogan.

“We pray for their families. But after we pray — after we get up off of our knees — we’ve got to demand change. We’ve got to demand justice,” state Attorney General Letitia James said an emotional church service in Buffalo on Sunday morning. “This was domestic terrorism, plain and simple.”

Among the dead was security guard Aaron Salter — a retired Buffalo police officer — who fired multiple shots at Gendron, Gramaglia said Saturday. A bullet hit the gunman’s armor, but had no effect. Gendron then killed Salter, before hunting more victims.

“He cared about the community. He looked after the store,” Yvette Mack, who had shopped at Tops earlier Saturday, said of Salter. “He did a good job you know. He was very nice and respectable.”

Also killed was Ruth Whitfield, 86, the mother of retired Buffalo Fire Commissioner Garnell Whitfield.

Buffalo Mayor Byron Brown told churchgoers that he saw the former fire official at the shooting scene Saturday, looking for his mother.

“My mother had just gone to see my father, as she does every day, in the nursing home and stopped at the Tops to buy just a few groceries. And nobody has heard from her,” Whitfield told the mayor then. She was confirmed as a victim later in the day, Brown said.

Katherine Massey, who had gone to the store to pick up some groceries, also was killed, according to the Buffalo News. The names of the rest of the victims hadn’t been released.

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

New York Gov. Kathy Hochul, a Buffalo native, called for the tech industry to take responsibility for their role in propagating hate speech in a Sunday interview with ABC.

“The CEOs of those companies need to be held accountable and assure all of us that they’re taking every step humanly possible to be able to monitor this information. How these depraved ideas are fermenting on social media – it’s spreading like a virus now,” she said, adding that a lack of oversight could lead to others emulating the shooter.

The mass shooting further unsettled a nation wracked with racial tensions, gun violence and a spate of hate crimes. A day before, Dallas police had said they were investigating shootings in the city’s Koreatown as hate crimes. The Buffalo attack came just 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.

Gendron, confronted by police in the store’s vestibule, put a rifle to his neck but was convinced to drop it. He was arraigned later Saturday on a murder charge, appearing before a judge in a paper gown.

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Balsamo reported from Washington.