The Northwest Airlines plane whose pilots flew past their destination by 150 miles had an older model cockpit voice recorder that only captures 30 minutes at a time, safety investigators said Friday.

National Transportation Safety Board investigators relying on the crucial black box to determine how Flight 188's two pilots overshot the Minneapolis airport and lost radio contact for more than an hour might be out of luck.

The Northwest pilots, who haven't yet been named but have been suspended pending the outcome of the investigation, were at 37,000 feet as they approached the city and fell out of touch with air traffic controllers who tried desperately to reach them.

Ground control feared the worst — a hijacking — and prepared to scramble fighter jets.

The pilots said they were distracted by a heated discussion of airline policy, but experts wonder if they fell asleep. The NTSB may have a hard time confirming what happened, as it's likely the voice recorder captured only the last 30 minutes of the flight — much of that time after pilots had realized their error and turned the plane back around.

Newer black boxes record for two full hours.

The pilots should have had several warnings that they were coming into Minneapolis and needed to bring the plane down for a landing: cockpit displays, repeated calls from air traffic controllers and twinkling city lights, to name a few.

Yet the pilots didn't discover their mistake Wednesday night until a flight attendant in the cabin contacted them by intercom, said a source close to the investigation who wasn't authorized to talk publicly and spoke on condition of anonymity.

By that time, the plane was over Eau Claire, Wis., and the pilots had been out of communication with air traffic controllers for over an hour. Pilots turned the plane back around and landed safely an hour and 15 minutes late, around 9:15 p.m. Wednesday.

Federal aviation authorities are investigating whether the Northwest pilots of the jet with 144 passengers aboard fell asleep at the controls, causing them to lose radio contact and overshoot Twin Cities International Airport.

The crew told authorities they were distracted during a heated discussion over airline policy, the Federal Aviation Administration said.

NTSB spokesman Keith Holloway said Thursday investigators hadn't yet questioned the pilots and didn't know whether it was possible they had fallen asleep and fatigue was to blame for the incident. Holloway called that theory "speculative."

The two black boxes were sent to Washington Friday for analysis. The pilots, whose names haven't been released, have been suspended from flying by the airline while it, too, investigates.

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."

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

The plane, en route from San Diego with a crew of five, passed over its destination of Minneapolis at 37,000 feet just before 9 p.m. EDT Wednesday.

Contact with controllers wasn't established until 14 minutes later, according to the National Transportation Safety Board, which is investigating the incident.

"It just doesn't make any sense," said Bill Voss, president of the Flight Safety Foundation in Alexandria, Va. "The pilots are saying they were involved in a heated conversation. Well, that was a very long conversation."

Ben Berman, an airline pilot and former chief of major accident investigations at the NTSB, said it becomes second nature for pilots to know when they need to begin landing preparations.

Those preparation should have begun when the flight was still 100 miles or more away from Minneapolis, he said. It would require a fairly dramatic event to lose track of that kind of awareness, he said.

Shop talk "pretty clearly wasn't all that was going on," Berman said.

The bright lights of Minneapolis should have alerted the pilots that they were over their destination, just as the dimmer lights of Eau Claire should have warned them they were in the wrong place, experts said.

While the passengers were apparently unaware what was happening as they passed their destination, police on the ground were preparing for the worst and the Air National Guard had put fighter jets on alert at two locations.

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.

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Andrea Allmon, who had been traveling from San Diego on business, didn't know anything was amiss.

"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. Allmon said she was "horrified" to learn what had happened and it was "unbelievable to me that they weren't paying attention. Just not paying attention."

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.

The FAA is updating rules governing how many hours commercial pilots may 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 and was heading out over open ocean before controllers raised the pilots. The captain was later diagnosed with sleep apnea.

Air traffic controllers in Denver had been in contact with the Northwest 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." That was just before 8 p.m.

Denver controllers notified their counterparts in Minneapolis, who also tried to reach the crew without success, Brown said.

The FAA had notified the military, which was ready to scramble as many as four Air National Guard fighter jets, but none took to the air.

The Associated Press contributed to this report.