The FBI and law enforcement officials in Columbia County, Oregon, arrested a teenager late last month who was allegedly planning to carry out a mass shooting involving explosives at a Washington state shopping mall.

In a news release Thursday, the FBI said the teen, whose name was not released, was arrested May 22 by deputies with the Columbia County Sheriff’s Office.

The FBI learned of “detailed and imminent” attack plans after they were reported to the agency just two days before the juvenile’s arrest.

After learning about the alleged planned attack, the FBI said it began working with its partners to identify the mastermind behind the threat. On May 20, the Columbia County teenager was identified as a suspect.

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According to the FBI, the teenager shared nihilistic violent extremist ideology, as well as the plans, in online chats.

The teenager was placed under court-authorized surveillance out of caution for the public, and, on May 22, a federal search warrant was executed, leading to the teen’s arrest.

The FBI said the suspect demonstrated the intention and means to carry out a plan that included details like the map of the Three Rivers Mall in Kelso, located more than two hours south of Seattle, and a route to follow.

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The plan allegedly involved the suspect using an improvised explosive device known as a chlorine bomb to incite panic before shooting mall patrons as they exited the movie theater and, ultimately, committing suicide at a pre-determined location at the mall, the FBI said.

The investigation determined the suspect seemed intent on following through with the plan.

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

While the FBI made initial contact with the teenager, the local sheriff’s office made the arrest on state charges.

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The Columbia County District Attorney’s office is prosecuting the case.

The FBI said it encourages the public to report suspicious behavior to law enforcement, adding that parents are also encouraged to engage in open dialog with their children about their online activity.