The man who allegedly shot 10 people dead and injured three more at a Buffalo, New York, supermarket Saturday likely planned out his racially motivated attack for months, authorities said Sunday.

Police pointed to the roughly 180-page manifesto the alleged shooter, 18-year-old Payton Gendron, posted before going on his rampage. The rambling text included a detailed plan of his assault on the Tops Friendly Market, which he drove to from several counties away, according to police.

“This defendant is accused of traveling to our area and targeting innocent people who were shopping for their groceries on a Saturday afternoon. I continue to pray for all affected by this horrific crime,” Erie County District Attorney John Flynn said Saturday. “I am committed to obtaining justice for the victims, their families and this community.”

“My office is working closely with the U.S. Attorney’s Office and our partners in law enforcement into potential terrorism and hate crimes. This is an active investigation and additional charges may be filed,” he added.

Gendron identified himself as a white supremacist in his manifesto, detailing at length his fears that white people are being replaced by other races.