An Oregon teen has been arrested in connection with an alleged mass shooting plot targeting the Three Rivers Mall in Kelso, in southwestern Washington, officials said.

The plot, described by one official as being “as serious as it gets,” involved a plan to shoot moviegoers at a Washington State mall as they fled an explosion.

A Columbia County teen planned to set off an explosive at the mall in Kelso off I-5 and then shoot people as they fled the movie theater there, the FBI said.