National Transportation Safety Board spokesman Keith Holloway said investigators were interviewing the pilot and co-pilot in person in Minneapolis. He would not provide additional details, but did say the NTSB would not comment on the substance of the discussions until Monday at the earliest.
Northwest Airlines is cooperating and doing its own internal investigation, said Chris Kelly, a spokesman for Northwest Airlines' parent company, Delta Air Lines Inc.
Air traffic controllers tried for more than an hour Wednesday night to contact the Minneapolis-bound flight, which later turned around and landed safely. First officer Richard Cole has said he and the captain were not sleeping or arguing in the cockpit, but hasn't explained their lapse in response and the detour. The Wall Street Journal reported on its Web site Sunday that the pilots planned to repeat their story to safety investigators during Sunday's interview.
The first officer on the flight made a public statement Friday that he and the captain were not sleeping or arguing in the cockpit, but did not explain their lapse in response and the detour.
"It was not a serious event, from a safety issue," pilot Richard Cole said late Friday 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 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."