Air traffic controllers and pilots had tried for more than an hour Wednesday night to contact the Minneapolis-bound flight. Officials on the ground alerted National Guard jets to prepare to chase the airliner, though none of the military planes left the runway.
The jet with 144 passengers aboard was being closely monitored by senior White House officials, White House spokesman Nick Shapiro told The Associated Press on Saturday. He didn't say if President Barack Obama was informed.
Many aviation safety experts and pilots say the most likely explanation is that the pilots fell asleep along their route from San Diego. NTSB spokesman Keith Holloway said fatigue and cockpit distraction are factors that will be looked into.
"We were not asleep; we were not having an argument; we were not having a fight," Cole said, but would not discuss why it took so long for him and the flight's captain, Timothy B. Cheney, of Gig Harbor, Wash., to respond to radio calls.
"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."
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 were in the process Saturday of scheduling interviews with the pilots, Holloway said, and audio from the cockpit voice recorder was downloaded at NTSB headquarters on Friday.
But they may not glean much from it. While new recorders retain as much as two hours of cockpit conversation and other noise, the older model aboard Northwest's Flight 188 includes just the last 30 minutes - only the very end of the flight after the pilots realized their error over Wisconsin.
The NTSB recommended a decade ago that airlines be required to have two-hour cockpit voice recorders. The standard has been 15- to 30-minute recorders.
Last year, the Federal Aviation Administration issued a rule requiring airplanes and helicopters seating 10 or more people to have the 2-hour audio recordings, but gave the industry time to comply. Aircraft made after March 2010 must come equipped with longer recorders, though many manufacturers have already been including them. Existing planes have until March 2012 to comply.
The FAA rule doesn't require cockpit video recordings, which the NTSB had also recommended. Pilots opposed the video recordings.
Northwest, which was acquired last year by Delta Air Lines, is also investigating the incident. Cheney and Cole have been suspended. Messages left at Cheney's home were not returned.
The pilots passed breathalyzer tests and were apologetic after the flight, according to a police report released Friday. Cheney and Cole had just started their work week and were coming off a 19-hour layover, the Minneapolis Star Tribune reported Saturday, citing an internal Northwest document it said was described to the newspaper.
The police report said that the crew indicated they had been having a heated discussion about airline policy.
AP Writers Joan Lowy in Washington and Amy Forliti in Minneapolis and AP Airlines Writers Joshua Freed in Minneapolis, Harry R. Weber in Atlanta and Dave Koenig in Dallas contributed to this report. Cain reported from Salem, Ore.
On the Net:
FlightAware.com tracking of Northwest Flight 188: http://bit.ly/2QV9hX
National Transportation Safety Board www.ntsb.gov