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An off-duty pilot who allegedly tried to crash an Alaska Airlines jet said he thought he was having a nervous breakdown, court documents show.

Joseph David Emerson also told police he had taken psychedelic mushrooms and that he had been depressed.

Mr Emerson pleaded not guilty to 83 counts of attempted murder in an Oregon court on Tuesday.

He was sitting in the cockpit of the flight behind the captain when the alleged incident occurred on Sunday.

Mr Emerson told the pilots "I am not okay" before reaching for the shutoff handles, according to the documents.

Had he been successful, a fire suppression system used to fight blazes in the jet engines would have been activated and cut off the supply of fuel.

The criminal complaint states that one pilot said he had to wrestle with Mr Emerson until he stopped resisting and was ushered out of the cockpit. The entire incident lasted about 90 seconds.

After being subdued, Mr Emerson said to flight attendants: "You need to cuff me right now or it's going to be bad" and later tried to reach for the emergency exit handle during the plane's descent, the documents say.

One flight attendant told investigators they had observed Mr Emerson saying "I messed everything up" and that he "tried to kill everybody".

During his police interview, Mr Emerson told investigators he had had a "nervous breakdown" and had not slept for 40 hours.

"I pulled both emergency shut off handles because I thought I was dreaming and I just wanna wake up," he said. "I didn't feel okay. It seemed like the pilots weren't paying attention to what was going on. They didn't… It didn't seem right."

"I'm admitting to what I did. I'm not fighting any charges you want to bring against me, guys," he added.

A spokesperson for the United States Attorney's Office told the ����ý that it was still being investigated whether Mr Emerson had been under the direct influence of a psychedelic substance at the time.

In addition to the allegations of attempted murder, Mr Emerson is also charged with 83 counts of reckless endangerment and one count of endangering an aircraft.

The flight was on its way from Everett, Washington, to San Francisco, California, with 80 passengers aboard. It was then diverted to Portland, Oregon.

In a recording of air traffic control communication, one of the pilots was heard saying: "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 issue in the back right now."

"I think he's subdued," the pilot added. He requested police presence "as soon as we get on the ground and parked".

Passenger Aubrey Gavello told that those on board had been unaware anything was wrong with the flight until the flight attendant announced that the plane needed to land immediately, later citing a medical emergency.

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

"So I really thought it was a serious medical emergency," she said.

Another passenger told the outlet the situation had been handled professionally and passengers had not been aware of the crisis.

On Monday, the Federal Aviation Administration sent guidance to US air carriers that the incident had not been "connected in any way, shape or form to current world events".

In a statement on Monday, the FBI confirmed it was investigating and said it could "assure the traveling public there is no continuing threat related to this incident".