Great News: Northwest Pilots Doze Off, Overshoot Airport by 150 Miles

OK, they claim they were embroiled in an argument and just happened to miss the Minneapolis airport. But the likelihood is they fell asleep. What other explanation is there when you have 78 minutes of radio silence?

Friday morning, federal aviation officials were investigating a mid-air mystery. Just what happened in the cockpit of a Northwest Airlines jet that flew over its destination and just kept going?

The flight from San Diego to Minneapolis did land safely. But Friday, the pilots faced many questions. The cockpit voice recorder and flight data recorder were on their way to Washington where investigators hoped to hear some answers.

Northwest Flight 188 left San Diego with 147 passengers on board, and at 37,000 feet it just kept going for 150 miles.

The NTSB reports the pilots said, "they were in a heated discussion over airline policy and they lost situational awareness."

But this agency is investigating another possibility: that the pilots fell asleep.

"This bears all the earmarks of a crew that's gone to sleep," John Nance said. "I would say actually that there's less than a one-percent chance that there was anything other rational explanation."

ABC News has learned the last radio contact with the plane was at 6:46 p.m. central time. Ten minutes later there was no response from the cockpit.

It was 8:14 when pilots reconnected. By then, the plane was over Wisconsin.

The aircraft safely landed more than one hour late at its intended destination: the Minneapolis Airport.

"If the sleepiness had overtaken them and it continued until the plane ran out of fuel, then yes, the passengers are most definitely in danger," Nance said.