He live streamed the attack before he surrendered outside of the store. Just before opening fire, Gendron posted his writings detailing his plans and motive, which law enforcement says was racial, AP reported.
The U.S. Justice Department is investigating it as a possible hate crime, the Post reported.
Gendron wrote that he planned the ambush after becoming infatuated with white supremacist thinking he found online.
In his writings he called himself a white supremacist, a fascist and an anti-Semite, CNN reported. He said he targeted the supermarket because it had the highest concentration of Black people in the area. He traveled three hours from his Conklin, New York, home to carry out the attack, The Washington Post reported.
He said he planned it in secret and had no help, but he did send an invitation to a small group of people to access his private writings about half an hour before he started shooting, the AP reported. Some accepted the invitation but officials are not sure how many actually read the documents and if anyone tried to alert the authorities.
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Gunman opens fire in Buffalo supermarket People hug outside the scene after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Police respond to the scene of a shooting in the parking lot of a supermarket where several people were killed in a shooting, Saturday, May 14, 2022 in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Police speak to bystanders while investigating after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket A body lays covered in the parking lot of a supermarket where several people were killed in a shooting, Saturday, May 14, 2022 in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online. (Mark Mulville/AP)
Gunman opens fire in Buffalo supermarket Police secure the area around a supermarket where several people were killed in a shooting, Saturday, May 14, 2022 in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online. (Mark Mulville/AP)
Gunman opens fire in Buffalo supermarket Buffalo Mayor Byron Brown speaks during a press conference after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Bystanders gather under an umbrella as rain rolls in after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Buffalo mayor Byron Brown takes a call after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket A bystander watches as police investigate after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Buffalo Mayor Byron Brown takes a moment of silence and prayer following a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Buffalo Police Commissioner Joseph Gramaglia speaks at a press conference after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Police investigate after a shooting at a supermarket, Saturday, May 14, 2022, in Buffalo, N.Y. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Buffalo Police respond to a shooting at Tops Friendly Market in Buffalo, N.Y., Saturday, May 14, 2022. (Carolyn Thompson/AP)
Gunman opens fire in Buffalo supermarket Police secure a perimeter after a shooting at a supermarket, Saturday, May 14, 2022, in Buffalo, N.Y. (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket A crowd gathers as police investigate after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. Multiple people were shot at the Tops Friendly Market. Police have notified the public that the alleged shooter was in custody. (AP Photo/Joshua Bessex) (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Police investigate after a shooting at a supermarket, Saturday, May 14, 2022, in Buffalo, N.Y. (AP Photo/Joshua Bessex) (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket A crowd gathers as police investigate after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. Multiple people were shot at the Tops Friendly Market. Police have notified the public that the alleged shooter was in custody. (AP Photo/Joshua Bessex) (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket People gather outside a supermarket where several people were killed in a shooting, Saturday, May 14, 2022 in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online (Derek Gee/The Buffalo News via AP) (Derek Gee/AP)
Gunman opens fire in Buffalo supermarket A crowd gathers as police investigate after a shooting at a supermarket on Saturday, May 14, 2022, in Buffalo, N.Y. Multiple people were shot at the Tops Friendly Market. Police have notified the public that the alleged shooter was in custody. (AP Photo/Joshua Bessex) (Joshua Bessex/AP)
Gunman opens fire in Buffalo supermarket Police secure an area around a supermarket where several people were killed in a shooting, Saturday, May 14, 2022 in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online (Derek Gee/The Buffalo News via AP) (Derek Gee/AP)
Gunman opens fire in Buffalo supermarket Police vehicles block off the street where at least 10 people were killed in a shooting at a supermarket, Saturday, May 14, 2022 in Buffalo, N.Y. Officials said the gunman entered the supermarket with a rifle and opened fire. Investigators believe the man may have been livestreaming the shooting and were looking into whether he had posted a manifesto online (Derek Gee/The Buffalo News via AP) (Derek Gee/AP)
