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. He also said an argument wasn't to blame. "All I'm saying is we were not asleep; we were not having a fight; there was nothing serious going on in the cockpit that would threaten the people in the back at all," he told The Associated Press in an interview at his home in Salem, Ore.
He declined to discuss what exactly happened but did insist "it was not a serious event, from a safety issue."
"I can't go into it, but it was innocuous."
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 report also said the lead flight attendant told police she was unaware of any incident during the flight.
The pilots, both temporarily suspended, are to be interviewed by NTSB investigators next week. The airline, acquired last year by Delta Air Lines, is also investigating. Messages left at both men's homes were not immediately returned.
The FAA said Friday letters had been sent informing the pilots they are being investigated by the agency and it is possible their pilot's licenses could be suspended or revoked.
Investigators don't know whether the pilots may have fallen asleep, but National Transportation Safety Board spokesman Keith Holloway said Friday that fatigue and cockpit distraction will be looked into. The plane's flight recorders were brought to the board's Washington headquarters.
Voss, the Flight Safety Foundation president, 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. Despite cloudy conditions, the lights of Minneapolis should have clued them in that they'd reached their destination, aviation experts said.
"It's probably something you would say never would happen if this hadn't just happened," Voss said.
The pilots were finally alerted to their situation when a flight attendant called on an intercom from the cabin. Two pilots flying in the vicinity were also finally able to raise the Northwest pilots using a Denver traffic control radio frequency instead of the local Minneapolis frequency.
On the ground, police and FBI agents prepared for the worst.
"When the aircraft taxied to the gate I was able to see the two white males in the seats of the flight crew, both were wearing uniforms consistent with Delta flight crew," said a police report, signed by an Officer Starch. "When the aircraft had stopped, the male seated in the pilot seat turned, looked at me and gave me two thumbs up and shook his head indicating all was OK."
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.
Officials suspect Flight 188's radio might still have been tuned to a frequency used by Denver controllers even though the plane had flown beyond their reach, said Church, the spokesman for the National Air Traffic Controllers Union. Controllers worked throughout the incident with the pilots of other planes, asking them to try to raise Flight 188 using the Denver frequency, he said.
Passenger Lonnie Heidtke said he didn't notice anything unusual before the landing except that the plane was late.
The flight attendants "did say there was a delay and we'd have to orbit or something to that effect before we got back. They really didn't say we overflew Minneapolis. ... They implied it was just a business-as-usual delay," said Heidtke, a consultant with a supercomputer consulting company based in Bloomington, Minn.
Once on the ground, the plane was met by police and FBI agents. Passengers retrieving their luggage from overhead bins were asked by flight attendants sit down, Heidtke said. An airport police officer and a couple other people came on board and stood at the cockpit door, talking to the pilots, he said.
"I did jokingly call my wife and say, 'This is the first time I've seen the police meet the plane. Maybe they're going to arrest the pilots for being so late.' Maybe I was right," Heidtke said. Delta said Friday it plans to give flight vouchers worth a few hundred dollars to each of the passengers.
AP Airlines Writer Joshua Freed and AP Writers Steve Karnowski and Amy Forliti in Minneapolis, Dave Koenig in Dallas, Harry Weber in Atlanta and Brad Cain in Salem, Ore., contributed to this report.
On the Net:
FlightAware.com tracking of Northwest Flight 188: http://bit.ly/2QV9hX
National Transportation Safety Board www.ntsb.gov