An Alaska Airlines passenger inside the cockpit attempted to seize control of a plane headed from a Seattle airport to San Francisco on Sunday, the airline said.

A pilot told air traffic controllers a person riding in the cockpit of flight 2059 attempted to turn off the aircraft's engines in flight. An airline's pilots, or pilots at other airlines, will occasionally ride a cockpit "jump seat" when traveling in an official capacity or commuting to another airport.

The passenger, who has not been named, was an off-duty pilot and has been arrested by Port of Portland Police Department, a source familiar with the matter told CNN.

The plane took off from Seattle Paine Field International Airport and was diverted to Portland International Airport because of the "credible security threat," the airline said.

"We've got the guy that tried to shut the engines down out of the cockpit, and he doesn't sound like he's causing any issues in the back right now," the pilot relayed to Seattle-area air traffic controllers in audio recorded by LiveATC.net. "I think he's subdued. Other than that, we want law enforcement as soon as we get on the ground and are parked."

The airline said in a statement on Monday the crew "reported a credible security threat related to an authorized occupant in the flight deck jump seat."

"Following appropriate FAA procedures and guidance from air traffic control, the flight safely diverted to Portland International Airport," Alaska Airlines said. The flight was operated by Horizon Air, a regional carrier owned by Alaska Airlines. "The event is being investigated by law enforcement authorities."

The FAA told CNN it is aware of the incident and supporting a local law enforcement investigation.

CNN has reached out to local and federal authorities for further information.