The Northwest Airlines plane involved was an Airbus A320
Investigations are under way in the US to find out how a plane heading from San Diego to Minneapolis overshot its destination by 150 miles (240km).
Contact with the Northwest Airlines plane was lost for an hour as it flew at 37,000ft, sparking hijack fears.
Investigators are examining data and voice recorders to establish why the pilots didn't hear air traffic control.
One of the pilots has denied they had fallen asleep, saying they had been distracted by a "heated discussion".
Flight 188, carrying 147 passengers, landed safely at Minneapolis after contact was resumed.
The National Transportation Safety Board (NTSB) 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, who told the FBI that "they were in a heated discussion over airline policy and lost situational awareness".
One of the pilots involved, Richard Cole, denied suggestions that he or his co-pilot had nodded off in the cockpit.
"I'll tell you this, neither of us was asleep," he told ABC News.
The data recorder could show if there was any manipulation of the controls on the flight deck.
The NTSB gave no indication of how long the analysis of the recorders could take.
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".
Passenger Andrea Allmon: 'The plane was swarmed by police'
Questions have been raised about how the pilots could have missed any warning signals - including city lights and cockpit displays showing that they were no longer on the right path.
The NTSB's former chief investigator into major accidents, Ben Berman said pilots learned to become instinctively aware of when they needed to start landing preparations and it would take a "fairly dramatic event" to distract them, AP reported.
The plane left San Diego at 1500 local time (2200 GMT) for what would normally be a three-hour journey.
Air traffic controllers lost radio contact with the plane approximately an hour before it was due to reach its destination of Minneapolis-St Paul International/Wold-Chamberlain Airport.
The plane missed its intended stop and continued on for a further 16 minutes before controllers managed to speak to the pilots.
The Federal Aviation Administration (FAA) contacted the military and two fighter jets were put on alert.
There were initial concerns the plane had been hijacked or had faced other problems and that was why the crew were not responding.
Northwest Airlines merged with Delta Air Lines in 2008.
In a statement, Delta Air Lines said the plane's pilot and co-pilot had been "relieved from active flying".
Oct 09: Police launch an inquiry after reports of Air India pilots and cabin crew coming to blows mid-air over sexual harassment claims
June 08: A Polish Boeing 737 narrowly misses hitting another aircraft over London after the wrong co-ordinates are entered into the flight computer
Feb 08: An internal Go! flight in Hawaii overshoots its landing by 15 miles after the two pilots fall asleep at the cockpit
It said a decision would be made about them once its own internal investigation and one by the FAA and the safety board were over.
Brent Bjorlin, who was on the flight, told the Minnesota Star Tribune newspaper that the passengers had not realised what had happened until they landed and security officials boarded the plane.
Fellow passenger Andrea Allmon said it was "unbelievable" that the pilots had allegedly not been paying attention.
"These guys are supposed to be paying attention to the flight. The safety of the passengers should be first and foremost," she said.
Doug Church, a spokesman for the National Air Traffic Controllers' Association, told the Wall Street Journal controllers at the airport began worrying about terrorism as a factor when they could not reach the crew.
He said when the controllers eventually made contact, to allay their fears the crew had been threatened or overpowered, they made the pilots prove they were still in control of the plane.
Once they were satisfied, the plane made its way back to the airport.
Were you a passenger on the plane? If you have any information you want to share with the BBC, you can do so using the form below:
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.