A teenager in Oregon planned to set off an explosive at a shopping mall in Washington State and shoot people as they fled the movie theater there, said law enforcement officials, one of whom said the plot “was as serious as it gets.”

The teenager, who is from Columbia County, Ore., and whose name and precise age were not released, was arrested on May 22 by deputies from the Columbia County Sheriff’s Office, according to the F.B.I.

Officials in the F.B.I.’s Portland, Ore., office said the attack was planned for the Three Rivers Crossing mall in Kelso, Wash., which is about 50 miles north of Portland. Representatives of the mall could not be immediately reached for comment on Saturday.

The detailed and imminent attack plans were reported to the F.B.I. on May 19, officials said at a news conference in Portland on Thursday. The agency worked with the local police to identify the suspect the next day.

Doug Olson, the special agent in charge at the Portland office of the F.B.I., did not disclose who tipped off the authorities, saying only that “if you see something, say something.”

The teenager was placed under court-authorized surveillance, and a federal search warrant was executed at his home on May 22.