voice recorder may still provide valuable information, because the pilots could have discussed the earlier events on the way back to Minneapolis after overshooting the airport.

The flight data recorder also could prove valuable because it would have recorded actions taken by the pilots during the 78 minutes they did not respond to repeated calls from air traffic controllers, the ex-investigator said.

Meanwhile, the North American Aerospace Defense Command, which scrambled fighter jets for the wayward plane but did not launch them, said it was reviewing procedures for launching the fighters to track potentially hijacked or suspicious aircraft.

At issue is the Federal Aviation Administration's apparent delay in notifying NORAD the Northwest jet was not in contact with controllers, according to a senior U.S. official directly familiar with the timeline ofthe incident.

Watch how the military is looking at a possible FAA delay

The official, who declined to be identified because the military and the FAA are reviewing the incident, said the FAA's request for military involvement came after the plane passed the Minneapolis-St. Paul airport. NORAD scrambled fighter jets at two locations. But as they approached the runway for takeoff, the FAA reported being back in contact with the Northwest flight, and the fighters stayed on the ground.

"My real question is why we did not know of the 'radio out' situation from the FAA sooner," the official said. "The FAA is also looking into that."

Since the September 11, 2001, attacks, NORAD has regularly launched fighter jets to track aircraft in unusual situations such as when they deviate from flight