they had passed" the airport, it added. After 78 minutes of radio silence, the pilots re-established radio contact with air traffic controllers, it said.

After landing at Minneapolis-St. Paul, both voluntarily underwent alcohol breath tests, which proved negative, the report said.

The safety board said its investigators interviewed the pilots separately Sunday in Minnesota for more than five hours combined. The investigation will include scrutiny of the flight and voice data recorders, it said.

An airline spokesman said Monday the company has sent the passengers on the plane $500 travel vouchers to compensate them for their inconvenience, and that the pilots have been suspended until the conclusion of the investigations.

The NTSB on Monday interviewed the three flight attendants who were on the plane, a spokesman for the Association of Flight Attendantssaid.

The lead flight attendant told officers she was unaware there had been an incident aboard, according to the report.

Police who met the wayward jet said the pilots were "cooperative, apologetic and appreciative."

The NTSB is hoping the plane's cockpit voice recorder either will confirm the pilot's account or provide evidence of another possible explanation, including whether the captain and first officer fell asleep.

Watch the co-pilot speak

The voice recorder is capable of recording only 30 minutes of audio, federal accident investigators said. The plane was in the air for another 45 minutes after radio contact was restored, meaning that if the recorder was working properly, anything the pilots would have said during the time they weren't answering radio calls would have been recorded over.

But a former accident investigator said the