The suspected shooter in the massacre at a Buffalo, New York, supermarket invited a small group of Discord users to join a private chat room to review his plan about 30 minutes before beginning his attack, The New York Times reported on Tuesday, citing a statement from the chat app.

Payton Gendron, the 18-year-old white suspect, is accused of killing 10 people with a semi-automatic rifle on Saturday at a grocery store in a predominantly Black neighborhood. Of the 13 people shot in the attack livestreamed on Twitch, 11 were Black in what the FBI has called an act of "racially motivated violent extremism."

In the months leading up to the attack, Gendron posted his plans for the attack on the chat app Discord, including images of himself posing with weapons and hand-drawn maps of the Tops Friendly Market where the attack occurred, The New York Times reported. The posts were visible only to Gendron until 30 minutes before the attack, when others joined the chat room, known as a server.

"What we know at this time is that a private, invite-only server was created by the suspect to serve as a personal diary chat log," a Discord spokeswoman told the Times.

"Approximately 30 minutes prior to the attack, however, a small group of people were invited to and joined the server. Before that, our records indicate no other people saw the diary chat log in this private server."

It wasn't immediately clear who was invited to the chat room, but none of them appeared to have alerted law enforcement to an  impending attack.

Discord didn't immediately respond to a request for comment.

Twitch has said it removed the livestream from its platform within two minutes of the violence starting at the grocery store. The alleged gunman also posted a 180-page hate-filled manifesto to 4chan, in which he spouted anti-immigrant views and described his plan to target Black people.