The attempt by an off-duty pilot to shut down the engines of a U.S. airliner in midflight highlights the threat that insiders pose to aviation safety with their ability to go where passengers are prohibited.

Events like the one Sunday on a Horizon Air jetliner are very rare, but they are potentially devastating.

The captain and co-pilot prevented disaster by subduing the off-duty Alaska Airlines pilot who — from his perfect vantage point in the cockpit jump seat — tried to cut fuel to the engines.

“We’re just lucky he wasn’t flying the plane when he decided to do this. What’s going to stop that?” said Jon Loffi, a longtime law enforcement officer who teaches aviation security at Oklahoma State University and wrote a paper on identifying insider threats.

There is something of a see-something, say-something mindset in aviation, where employees are encouraged to voluntarily report anything that raises a safety concern, including suspicious behavior of colleagues. It’s not clear how often that happens, however — if ever.

The pilot who was arrested after Sunday’s flight didn’t appear to raise alarms. Neighbors and fellow members of a flying club where he was an instructor described him as a great guy who was obsessed with safety.

Pilots are required to undergo psychological screening as part of their regularly scheduled medical exams. The pilot involved in Sunday’s incident got his most recent exam in September, according to Federal Aviation Administration records.

However, the pilot, Joseph David Emerson, told police that he had been depressed for about six months — starting long before his last medical exam — and was having a “nervous breakdown,” according to an FBI agent’s affidavit.

During FAA-required exams, pilots are supposed to go through an interview and report whether they have received psychiatric treatment and what medicines they take. The FBI agent’s affidavit didn’t indicate whether Emerson was being treated for depression.

The FAA says its approved medical examiners are trained to gauge a pilot’s mental health.

Airlines frequently conduct their own interviews that include a psychological evaluation, “but most of those are to determine whether the pilot is a good fit for the company, not whether they are mentally unstable,” said Jeffrey Price, an aviation-security expert at Metropolitan State University of Denver.

The Horizon close call could result in the re-examination of the practice of letting off-duty pilots or other airline employees sit in the cockpit jump seat.

“I’m afraid this will put that privilege in jeopardy now,” said Ross Aimer, a retired airline pilot who is now CEO of an aviation-consulting firm. “This is obviously an extremely important issue, and they have to quickly come up with an answer.”

Price said airlines must approve people who sit in the jump seat, but the captain can deny access.

“Our flight crew vetting system is just that — it’s based on trust,” he said. “We have to trust personnel to a certain extent, or the system doesn’t work.”

Loffi, the Oklahoma State professor, said banning pilots from the jump seat would be silly because a pilot intent on crashing the plane could simply wait until their turn to operate a flight.

Investigators concluded that is what happened on board a Germanwings plane in 2015. They determined that the co-pilot deliberately crashed the plane in the French Alps. The man had been treated for suicidal tendences but did not tell the airline.

A FedEx pilot riding on a cargo plane tried to kill the crew and crash the plane in 1994 but was stopped by the on-duty pilots. In 1987, a fired employee who still had his security badge hijacked a Pacific Southwest Airlines jet, which crashed in California, killing everyone on board.

On rare occasions, other employees including a mechanic have been accused of sabotaging planes.

Loffi said pilots could be subjected to more rigorous psychological examinations, as is often done with police officers, but that’s not foolproof either.

“I think we’re doing a pretty good job of managing the insider threat,” he said. “How often does this happen? Darn seldom. And it’s just so hard to predict this kind of behavior.”

Transportation Secretary Pete Buttigieg said the FAA will look into “any safety considerations” that come out of investigations of the Horizon Air incident.

Security experts say it would be difficult if not impossible to stop every determined criminal or terrorist who targets aviation. They say there are steps that could make such an attack more difficult, but they would come at a cost of time and money.

One possibility is to make airline and airport employees go through the same type of security checkpoints as passengers. Currently many of those insiders can go behind security perimeters with the right credentials.

Federal law requires that people seeking to work in secure areas of an airport must pass a criminal-records check and a “security threat assessment” that includes checking their names against a terrorism watch list. The vetting is conducted by the Transportation Security Administration.

The off-duty Alaska Airlines pilot who was arrested Sunday joined Horizon as a first officer or co-pilot in 2001. In 2012, he went to work at Virgin America, which was bought by Alaska in 2016. Emerson was promoted to captain in 2019.

Alaska Airlines said Emerson met all requirements for medical exams “and at no point were his certifications denied, suspended or revoked.”

Alaska said Tuesday that Emerson was “relieved from all duties” on Sunday, the day of the flight. His next flight had been planned for Thursday. The airline indicated it was talking to the pilots’ union “regarding his employment status.”

On Tuesday, Emerson was being held in the Multnomah County, Oregon, jail on one federal count of interfering with a flight crew and 83 state counts of attempted murder, a felony, 83 counts of misdemeanor reckless endangerment, and one count of endangering an aircraft, a felony. He was due to be arraigned Tuesday in Portland.

Emerson’s neighbors in Pleasant Hill, California, a suburb about 30 miles east of San Francisco, describe him as friendly, and officials of a local flying club say he is meticulous about safety.

“No sign he was off. Nothing,” Karen Yee, a neighbor whose grandchildren play with Emerson’s two elementary-school kids, told The (San Jose) Mercury News. “He is everything you would want to have in a good neighbor. We see him over the fence and on walks. Great guy. Great family.”

Allen Scott, former president of a flying club where Emerson served as a flight instructor several years ago, told the San Francisco Chronicle he has flown with Emerson and he was “fanatical” about safety.