The Minneapolis skyline rises through the rain as an arriving Northwest Airlines jet taxis Friday at Minneapolis-St. Paul International Airport. Investigators are looking into a Northwest flight bound for Minneapolis from San Diego that overflew the airport by 150 miles.

Washington ? Were the pilots distracted? Catching up on their sleep? Federal investigators struggled to determine what the crew members of a Northwest Airlines jetliner were doing at 37,000 feet as they sped 150 miles past their Minneapolis destination and military jets readied to chase them. Unfortunately, the cockpit voice recorder may not tell the tale.

A report released late Friday said the pilots passed breathalyzer tests and were apologetic after Wednesday night’s amazing odyssey. They said they had been having a heated discussion about airline policy. But aviation safety experts and other pilots were frankly skeptical they could have become so consumed with shoptalk 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.

One of the two pilots, first officer Richard I. Cole, said that wasn’t the case.

“I can assure you none of us was asleep,” Cole told ABC News. He declined to comment further except to say, “I am not doing very good.”

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.

They had flown through the night with no response as air traffic controllers in two states and pilots of other planes over a wide swath of the mid-continent tried to get their attention by radio, data message and cell phone. On the ground, concerned officials alerted National Guard jets to go after the airliner from two locations, though none of the military planes got off the runway.

With worries about terrorists still high, even after contact was re-established, air traffic controllers asked the crew to prove who they were by executing turns.

“Controllers have a heightened sense of vigilance when we’re not able to talk to an aircraft. That’s the reality post-9/11,” said Doug Church, a spokesman for the National Air Traffic Controllers Association.

A report released by airport police Friday identified Cole, of Salem Ore., and the flight’s captain, Timothy B. Cheney, of Gig Harbor, Wash. The report said the men were “cooperative, apologetic and appreciative” and volunteered to take preliminary breath tests that were zero for alcohol use.

The pilots, both temporarily suspended, are to be interviewed by NTSB investigators next week.

Some pilots say cockpit catnaps happen.

“Pilots on occasion do take controlled naps,” said Barry Schiff, an aviation safety consultant and retired pilot for erstwhile TWA. “So this is not without precedent.”

Although the Federal Aviation Administration prohibits pilots from snoozing in the cockpit, several airline pilots say they are surprised such napping mishaps haven’t happened more often, considering longer work schedules for pilots and advances in aviation that make planes easier to fly.

If investigators conclude that the Northwest pilots were snoozing at 37,000 feet, several current and retired pilots say it wouldn’t be a surprise.

“Fatigue is a real problem,” said Sam Mayer, an American Airlines pilot and spokesman for the Allied Pilots Association, the union that represents 11,500 American Airline pilots. “I don’t know what happened (in Minneapolis), but I wouldn’t be surprised if they were asleep.”