New York authorities on Sunday were investigating how a white 18-year-old, who the governor said had been on the radar of authorities since high school, was able to shoot 10 people to death in a Black neighborhood grocery store.
The suspect, Payton Gendron, surrendered to police on Saturday at the Buffalo, New York, store after what authorities called an act of "racially motivated violent extremism." He apparently publicized a racist manifesto on the internet.
The Buffalo shooting follows other racially motivated mass murders in recent years, including a Pittsburgh synagogue attack that left 11 congregants dead in October 2018, and the Atlanta spa shootings in March 2021 in which a white man killed eight people, targeting Asians.
Authorities said Gendron was armed with an assault-style rifle and drove to Buffalo from his home several hours away to launch the afternoon attack, which he broadcast in real time on social media platform Twitch, a live video service owned by Amazon.com.
Eleven of the 13 people struck by gunfire were Black, officials said. The two others were white. The racial breakdown of the dead was not made clear.
A 180-page manifesto circulating online on Saturday, believed to have been authored by Gendron, outlined "The Great Replacement Theory" - a racist conspiracy theory that white people are being replaced by minorities in the United States and other countries.
"This manifesto tells everything to us and that is what's so bone-chilling about it," New York Governor Kathy Hochul told CNN on Sunday.
Another document circulating online that appeared to have been written by Gendron sketched out a to-do list for the attack, including cleaning the gun and testing the livestream.
A spokesperson for the Erie County district attorney's office declined to comment on the documents.
Hochul told ABC News on Sunday that Gendron had been on authorities' radar "with respect to something he wrote in high school," without elaborating.
The governor said an investigation into the massacre would focus on how Gendron managed to get away with it when he was known to authorities and presented a threat.
"I want to know what people knew and when they knew it," she said.
Hochul told reporters she was dismayed that the suspect managed to live-stream his attack on social media, which she blamed for hosting a "feeding frenzy" of violent extremist ideology.
"The fact that that can even be posted on a platform is absolutely shocking," Hochul said. "These outlets must be more vigilant in monitoring social media content."
Twitch said in a statement that it removed the livestream less than two minutes after it started and was working to ensure no other accounts re-posted the content. Hochul said it should have been taken down "within a second."
Screenshots of the broadcast were posted on social media, including some that appeared to show the shooter holding a gun and standing over a body in the grocery store.
Gendron, who is from the town of Conklin, New York, near the Pennsylvania border, was arraigned hours after the shooting in state court on first-degree murder charges, which carry a maximum penalty of life in prison without parole, said Erie County District Attorney John Flynn. New York has no capital punishment.
Flynn said the judge also ordered Gendron to remain in custody without bail and to undergo a "forensic examination." Gendron was scheduled to return to court on May 19.
Gendron entered a plea of not guilty.
Hochul said the firearm used in the killings was legally purchased but had been illegally modified with a high-capacity magazine.
On Sunday, several dozen community members were holding a vigil for the victims outside the Tops grocery store.
Tyrell Ford, one of the vigil organizers with Voice Buffalo, a social justice nonprofit, said the shooting had rattled a "city of good neighbors."
"I'm still trying to process it. There's so much trauma in the Black community and this is the time to start the process of grieving," Ford said.
Sharon Doyle, a 55-year-old security guard with Erie County Public Library, led a chant of "Black Lives Matter, my life matters," at the vigil.
"We all go in this Tops. I was scared to even go to Walmart last night," Doyle said. "I have to go to work tomorrow and I'm terrified."
Stephen Belongia, the FBI special agent in charge of the bureau's Buffalo field office, said the attack would be investigated both as a hate crime and as an act of "racially motivated violent extremism" under federal law.
U.S. President Biden decried the shooting as "abhorrent to the very fabric of this nation" in a statement issued late Saturday.
Authorities said the teenager had come close to taking his own life before he was arrested. When confronted by officers at the store, the suspect held a gun to his own neck, but they talked him into dropping the weapon and surrendering.