The plane landed safely Wednesday evening, and no one was hurt. But federal officials on Thursday began investigating whether pilot fatigue was a factor.
Keith Holloway, a spokesman for the National Transportation Safety Board, said the agency does not know if the crew fell asleep, calling that idea "speculative."
Flight 188, an Airbus A320, was flying from San Diego to Minneapolis with 144 passengers and five crew. The pilots dropped out of radio contact with controllers just before 7 p.m. CDT, when they were at 37,000 feet.
The jet flew over the airport just before 8 p.m. and overshot it before communications were re-established at 8:14 p.m, the NTSB said.
The Federal Aviation Administration said the crew told authorities they became distracted during a heated discussion over airline policy and lost track of their location.
The FAA notified the military, which put Air National Guard fighter jets on alert at two locations. As many as four planes could have been scrambled, but none ever took to the air.
"After FAA re-established communications, we pulled off," said Michael Kucharek, a North American Aerospace Defense Command spokesman.
As of Thursday afternoon, NTSB investigators had not yet examined the cockpit voice recorder and flight data recorder, which were being sent to the NTSB lab in Washington for analysis. He said the agency was also seeking to interview the pilots, but had not scheduled a meeting.
Anthony Black, a spokesman for Delta, which acquired Northwest last year, said the two pilots have been suspended from flying while the airline conducts an internal investigation. He refused to name them or give further details on their background or what happened in the air.
FAA spokeswoman Laura Brown said air traffic controllers in Denver had been in contact with the pilots as they flew over the Rockies. 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. Controllers and the pilots finally resumed communication when the plane was over Eau Claire, Wis.
"Radar controllers were the whole time trying to make audio contact with that plane," said Tony Molinaro, an FAA spokesman in Chicago.
It was not clear who initiated communications when contact finally was made, Brown said.
After the plane landed in Minneapolis-St. Paul International Airport, two airport police officers boarded the plane at the gate, which authorities said is standard procedure after a crew loses communication with air traffic controllers.
Kelly Regus, a spokeswoman for the Delta branch of the Air Line Pilots Association, declined to comment.
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Associated Press writers Martiga Lohn and Brian Bakst in St. Paul, David Koenig in Dallas, and Anne Gearan in Washington contributed to this report.
