Two Northwest Airlines pilots who overshot their destination by 150 miles (240 kilometers) before turning back should have had numerous warnings as they approached and passed Minneapolis: cockpit displays, controllers trying repeatedly to reach, the city lights twinkling below.

Yet the pilots didn't discover their mistake 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, Wisconsin, and the pilots had been out of communication with air traffic controllers for over an hour.

The crew told authorities they were distracted during a heated discussion over airline policy, the Federal Aviation Administration said. But federal officials are investigating whether pilot fatigue might be to blame.

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. The pilots have been suspended from flying by their airline while it, too, investigates.

The plane, en route from San Diego with 144 passengers and a crew of five, passed over its destination of Minneapolis at 37,000 feet (11,280 meters) just before 9 p.m. EDT Wednesday (0100 GMT Thursday). Contact with controllers wasn't established until 14 minutes later, according to the National Transportation Safety Board, which is investigating the incident.

The first officer of the Northwest Airlines jet that missed its destination by 150 miles says there was no fight in the cockpit, neither he nor the captain had fallen asleep and the passengers were never in any danger.

But in an interview with The Associated Press two days after he and a colleague blew past their destination as air traffic controlled tried frantically to reach them, pilot Richard Cole would not say just what it was that led to them to forget to land Flight 188.

"It was not a serious event, from a safety issue," Cole said in front of his Salem, Ore., home. "I would tell you more, but I've already told you way too much."

Air traffic controllers and pilots tried for more than an hour Wednesday night to contact Cole and the flight's captain, Timothy B. Cheney, of Gig Harbor, Wash., using radio, cell phone and data messages. On the ground, concerned officials alerted National Guard jets to prepare to chase the airliner from two locations, though none of the military planes left the runway.

Cole would not discuss why it took so long for the pilots to respond to radio calls, "but I can tell you that airplanes lose contact with the ground people all the time. It happens. Sometimes they get together right away; sometimes it takes awhile before one or the other notices that they are not in contact."

A police report released Friday said the pilots passed breathalyzer tests and were apologetic after the flight. The report also said that the crew indicated they had been having a heated discussion about airline policy.

But aviation safety experts and other pilots were deeply skeptical they could have become so distracted by shop talk that they forgot to land an airplane carrying 144 passengers. The most likely possibility, they said, is that the pilots simply fell asleep somewhere along their route from San Diego.

"It certainly is a plausible explanation," said Bill Voss, president of the Flight Safety Foundation in Alexandria, Va.

Unfortunately, the cockpit voice recorder may not tell the tale.

New recorders retain as much as two hours of cockpit conversation and other noise, but the older model aboard Northwest's Flight 188 includes just the last 30 minutes — only the very end of Wednesday night's flight after the pilots realized their error over Wisconsin and were heading back to Minneapolis.

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Voss said a special concern was that the many safety checks built into the aviation system to prevent incidents like this one — or to correct them quickly — apparently were ineffective until the very end. Not only couldn't air traffic controllers and other pilots raise the Northwest pilots for an hour, but the airline's dispatcher should have been trying to reach them as well. The three flight attendants onboard should have questioned why there were no preparations for landing being made. Brightly lit cockpit displays should have warned the pilots it was time to land.