The off-duty pilot accused of trying to crash an Alaska Airlines passenger plane allegedly tried to shut down the engines "by engaging the Engine Fire Handle," according to the airline.

The fire suppression system on the plane consists of a T-valve handle for each engine, and if those handles are fully deployed, a valve in the wing closes to shut off fuel to the engine, Alaska Airlines said.

The "quick reaction of our crew to reset the T-handles ensured engine power was not lost," Alaska Airlines said.

At the time of the Sunday incident, off-duty captain Joseph David Emerson was sitting in the flight deck jump seat, which is in the cockpit, Alaska Airlines said. Emerson was heard in the moments prior to the incident saying something similar to, "I’m not right," preliminary information obtained by investigators indicated, according to a federal official briefed on the probe.

The flight was en route from Everett, Washington, to San Francisco when it diverted to Portland, Oregon, the airline said. Emerson was scheduled to be on a flight crew of a 737 leaving San Francisco, according to a federal official.

Emerson was taken into custody in Portland and faces charges, including 83 counts of attempted murder, according to officials. He is due in court on Tuesday afternoon.

There were 80 passengers and four crew members on the flight, according to Alaska.

"We didn't know anything was happening until the flight attendant got on the loudspeaker and made an announcement that there was an emergency situation and the plane needed to land immediately," passenger Aubrey Gavello told ABC News, "… About 15 minutes later, she got back on and said that there was a medical emergency."

Passenger Alex Wood said the pilot announced that "there was a disturbance in the cockpit."

Gavello said she heard a flight attendant tell the suspect, "We're going to be fine, it's OK, we'll get you off the plane."

"After we did land and the gentleman was escorted off, the flight attendant got back on the speaker and said, plain and simple, 'He had a mental breakdown. We needed to get him off the plane immediately,'" Gavello said.

Alaska said Emerson joined the carrier as a Horizon First Officer in 2001. He then left the airline in 2012 to join Virgin America as a pilot.

Emerson returned to Alaska in 2016 when the carrier acquired Virgin America and he became a Captain with Alaska in 2019, the airline said.

Alaska said during Emerson's time with the carrier he "completed his mandated FAA medical certifications in accordance with regulatory requirements, and at no point were his certifications denied, suspended or revoked."

The event is being investigated by law enforcement, the airline said. The FBI said it "can assure the traveling public there is no continuing threat related to this incident."

The Federal Aviation Administration said in a statement to carriers that the incident is "not connected in any way shape or form to current world events."