(The Hill) — The suspect in Saturday’s mass shooting in Buffalo, New York, set up a chatroom on Discord just before he shot and killed 10 people, according to an invitation for the room reviewed by The Washington Post.

The Post obtained a copy of an invitation from shooting suspect Payton Gendron titled “Happening: This is not a drill” right before 13 people were shot at a Tops grocery store.

A person familiar with the matter told the outlet that 15 people joined the room, prompting investigators to look into the possibility that Gendron was encouraged by online friends. 

The discovery means more people were able to watch the shooting than initially believed, with authorities aware shortly after the shooting that it had been livestreamed on Twitch.

The Post says a review of the shooter’s Discord server shows a number of racist messages and planning for the mass shooting going back as many as six months. 

Gendron is accused of espousing racist conspiracy theories and driving hours to commit the shooting in a predominantly Black neighborhood. Eleven of the 13 victims were Black. 

Erie County Sheriff John Garcia has called the shooting a “straight-up racially motivated hate crime.”

The source told the Post that authorities are unaware of what the 15 Discord members who accepted the shooter’s invitation did after they joined his server. 

Discord issued a statement on Wednesday saying they are working with law enforcement to assist the investigation. 

The Hill has reached out to Discord and the Erie County Sherriff’s Office for comment.