Federal aviation authorities are investigating whether the pilots of a Northwest Airlines passenger jet fell asleep at the controls, causing them to lose radio contact and overshoot the Minneapolis airport by 150 miles.

The two pilots discovered their mistake after about an hour and turned the plane around, landing safely Wednesday evening. Air traffic controllers on the ground couldn't make contact with them as they flew past their destination.

Passengers didn't know anything was wrong until police swarmed the aircraft after it had touched down. No one was injured.

The Federal Aviation Administration said the crew told authorities they became distracted during a heated discussion over airline policy and lost track of their location.

But federal officials are investigating whether pilot fatigue might also have played a role.

The National Transportation Safety Board does not yet know if the crew fell asleep, spokesman Keith Holloway said, calling that idea "speculative." The black boxes were due to arrive in Washington for analysis sometime Friday.

Amy Kieffer was sitting in the sixth row of Flight 188 and said she thought they were running late — but then a bizarre announcement was made from the cockpit.

"The captain came on and said, 'After some back and forth bickering, we should be landing in 15 or 20 minutes,'" Kieffer told MyFoxTwinCities.com.

The plane touched down at Twin Cities International Airport an hour and 15 minutes after its scheduled arrival time.

The pilots didn't become aware of their situation until a flight attendant contacted them through an intercom from the cabin to the cockpit, said a source familiar with the investigation who wasn't authorized to speak publicly and asked not to be named.

Flight 188, an Airbus A320, was flying from San Diego to Minneapolis with 144 passengers and five crew. The pilots dropped out of radio contact with controllers just before 7 p.m. CDT, when they were at 37,000 feet.

The jet flew over the airport just before 8 p.m. and overshot it before communication was re-established at 8:14 p.m, the NTSB said.

The FAA notified the military, which put Air National Guard fighter jets on alert at two locations. As many as four planes could have been scrambled, but none took to the air.

"After FAA re-established communications, we pulled off," said Michael Kucharek, a North American Aerospace Defense Command spokesman.

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Several aviation experts have said they believe there is only one explanation for what happened.

"I think these guys fell asleep," Robert Mark, former airline pilot and editor of industry blog Jetwhine.com, told Fox News on Friday.

"The only reason we even heard from these guys is because the flight attendants banged on the door. If that hadn't happened — and thank God for the flight attendants — this could have been a much bigger disaster."

Andrea Allmon, who had been traveling from San Diego on business, said no one on the plane knew anything was wrong until the end of the flight.

"Everybody got up to get their luggage and the plane was swarmed by police as we were getting our bags down from the overhead bins," she said.

She said they were kept on the plane briefly while police talked to the crew, then allowed off. She said she was "horrified" to learn what had happened.

"When I do my job I do my job," she said. "These guys are supposed to be paying attention to the flight. The safety of the passengers should be first and foremost. (It's) unbelievable to me that they weren't paying attention. Just not paying attention."

As of Thursday afternoon, NTSB investigators had not yet examined the cockpit voice recorder and flight data recorder, which were being sent to Washington for analysis. Holloway said the agency was also seeking to interview the pilots, but had not scheduled a meeting.

A320s are equipped with a phone-like device that airline dispatchers can use to contact the crew, said Shirley Phillips, a former simulator instructor for US Airways who has flown the A320.

"It's fairly loud," she said. "There's lots of whistles and bells and things in the Airbus that all signify different things, but it has a pretty distinctive sound to it. I wouldn't say that you would mistake it for anything else."

In any case, she said the cockpit voice recorder should prove whether the pilots were arguing, as they claim.

"It literally records when they sneeze. It's going to record all the noise that goes on there."

One of the two pilots should have been paying attention to the radio, said Ronald Carr, a former Air Force and American Airlines pilot who teaches flight physiology at Embry-Riddle Aeronautical University. But he added that "sometimes you can have such heated discussions and get so distracted that you lose situational awareness, and when you're traveling seven miles a minute, that can happen pretty quick."

The two pilots have been suspended from flying while Delta Air Lines Inc. conducts an internal investigation, said Anthony Black, a spokesman for the Atlanta-based airline, which acquired Northwest last year. He refused to name them or give further details on their background or what happened in the air.

Air traffic controllers in Denver had been in contact with the pilots as they flew over the Rockies, FAA spokeswoman Laura Brown said. But as the plane got closer to Minneapolis, she said, "the Denver center tried to contact the flight but couldn't get anyone." Denver controllers notified their counterparts in Minneapolis, who also tried to reach the crew without success, Brown said. Controllers and the pilots finally resumed communication when the plane was over Eau Claire, Wis.

"Radar controllers were the whole time trying to make audio contact with that plane," said Tony Molinaro, an FAA spokesman in Chicago. He said he was not aware of controllers diverting any other flights, which was unnecessary because the Northwest jet was flying high enough to safely avoid planes approaching Minneapolis-St. Paul International Airport.

It was not clear who initiated communications when contact finally was made, Brown said.

After the plane landed, two airport police officers boarded the plane at the gate, which authorities said is standard procedure after a crew loses communication with air traffic controllers.

Kelly Regus, a spokeswoman for the Delta branch of the Air Line Pilots Association, declined to comment.

The Federal Aviation Administration is updating decades-old rules governing how long commercial pilots can fly and remain on duty. The NTSB also cautioned government agencies this week about the risks of sleep apnea contributing to transportation accidents.

In January 2008, two go! airlines pilots fell asleep for at least 18 minutes during a midmorning flight from Honolulu to Hilo, Hawaii. The plane passed its destination before controllers raised the pilots, who landed safely. The captain was later diagnosed with sleep apnea.

The Associated Press contributed to this report.