An inquiry has begun after a United States plane heading from San Diego to Minneapolis missed the airport it was meant to land at by 240 kilometres.
Contact with the Northwest Airlines plane was lost for an hour as it flew at 37,000 feet, sparking hijack fears.
The crew say were distracted by a heated discussion, the BBC reports.
The National Transportation Safety Board said the cockpit voice and data recorders had been sent to Washington DC, where they are being analysed.
It is hoped the voice recordings will provide evidence of the conversations between the two pilots.
Board spokesman Keith Holloway told the Associated Press news agency that reports in the media that the pilots may have fallen asleep were "speculative" but the investigation would look at "fatigue issues".
The plane, carrying 147 passengers, landed safely at Minneapolis after contact was resumed.
The pilot and co-pilot have been relieved from active flying.