The 19-year-old accused of gunning down 17 people at his former high school admitted he carried out the shooting rampage, authorities said in court papers filed Thursday.

Police wrote that Nikolas Cruz, who has been charged with 17 counts of premeditated murder, told officers that he walked into Marjory Stoneman Douglas High School on Wednesday wielding an AR-15 rifle and began shooting students in the hallways and outside on the school’s grounds. His victims included athletic director, athletes, football coach and students, reports The Washington Post.

Once students began to flee the carnage, Cruz dropped his rifle and vest packed with additional ammunition “so he could blend into the crowd,” an officer wrote in a probable-cause affidavit. Cruz had taken an Uber to the school, officials wrote, so he fled on foot along with those running from the scene. An officer found him not long afterward walking on a residential street.

The revelations came as police vowed to make sure that “justice is served” after one of the country’s deadliest school shootings. A day after the Valentine’s Day shooting rampage, authorities had not publicly announced a motivation for the carnage, but they were digging into elements of Cruz’s troubled past, including a pattern of disciplinary issues and unnerving behaviour.

Also on Thursday, FBI investigators were pursuing information suggesting that Cruz might have been associated with a Florida-based white supremacist group. But agents were still trying to determine the extent of his involvement with the group, if any, according to a law enforcement official who asked not to be identified discussing an ongoing investigation.

It was not immediately clear what connection might exist between the shooting rampage and Cruz’s possible link to the group. Broward County Sheriff Scott Israel said that authorities were still looking into whether he was indeed a member of the group.