An off-duty pilot for Alaska Air Group Inc. is in custody for allegedly trying to shut off power to a plane’s engines during a Horizon Air flight with passengers on board.

The unidentified person was riding in a cockpit jump seat on the Oct. 22 flight when they “unsuccessfully attempted to disrupt the operation of the engines,” Alaska said Monday in a statement. “The Horizon captain and first officer quickly responded, engine power was not lost and the crew secured the aircraft without incident.” Horizon is a regional carrier owned by Alaska.

The flight from the Seattle area to San Francisco was diverted to Portland, Oregon, and the passengers were re-booked on a later flight. The incident is being investigated by law enforcement authorities, including the FBI and local police.

The US Federal Aviation Administration said in a separate statement that it is “engaged with Alaska and Horizon airlines and is supporting law enforcement investigations.”