A Columbia County teenager was arrested after the FBI accused them of planning a bombing and mass shooting of a Washington mall.

The FBI Portland Field Office, which did not identify the suspect as they are a minor, said the teen shared “nihilistic violent extremist ideology” and plans for the explosive attack and shooting online. The suspect had already been under court-authorized surveillance due to public safety concerns, according to authorities.

The regional FBI office said the suspect had a route they intended to follow at the Three Rivers Mall in Kelso, Washington, and a plan to use an “improvised explosive device commonly known as a chlorine bomb to incite panic.” The plans then allegedly showed the teen would shoot mallgoers as they exited the on-site moving theater before killing themself “at a predetermined location in the mall.”

“This plot was as serious as it gets,” FBI Portland Special Agent in Charge Douglas A. Olson said in a release. “We, along with our partners, moved swiftly to interrupt this violent plan and to protect our community.”

Details of the planned attack were reported to the FBI on May 19, and the suspect was identified May 20. Columbia County Sheriff’s deputies arrested the teen May 22 in Columbia County. The Columbia County District Attorney’s Office will be prosecuting the case.