A passenger flight from San Diego, California, to Minneapolis, Minnesota, overshot its destination airport by about 150 miles Wednesday, and federal investigators are looking into whether the pilots had become distracted, as they claimed, or perhaps fallen asleep.

Air traffic controllers lost radio communication with the Northwest Airlines Airbus A320, carrying 147 passengers and an unknown number of crew, when it was flying at 37,000 feet, according to the National Transportation Safety Board. There was no communication with Flight 188 for more than an hour as it approached the airport, the board said.

When air traffic controllers finally made contact with the pilot, his answers were so vague that controllers feared the plane might have been hijacked, according to a source familiar with the incident.

The controllers in Minneapolis ordered the pilot to make a series of unnecessary maneuvers to convince them the pilots were in control of the flight, the source said, adding that fighter jets were poised in Madison, Wisconsin, but were never deployed.

Controllers tracked the aircraft on radar as it flew over its intended destination — Minneapolis-St. Paul International/Wold-Chamberlain Airport — and continued northeast for about 150 miles over the next 16 minutes. The airport’s controllers then re-established communication with crew members, who said they had become distracted, the safety board said.

“The crew stated they were in a heated discussion over airline policy and they lost situational awareness,” the board said in a news release.

A federal official, who asked not to be identified, told CNN that air traffic controllers in the Denver, Colorado, area had communicated with the pilot, but the pilots were “nonresponsive” during a subsequent communication. The plane was handed off to controllers in Minneapolis as a NORDO, the designation for “no radio communications.”

The Federal Aviation Administration contacted the airline and had its dispatcher try to reach the pilots, the federal official said.

Doug Church, spokesman for the National Air Traffic Controllers Association, said numerous controllers were involved in efforts to contact the plane, including text messages, and that “concern escalated” as the pilot neared the airport “without making any effort to descend.”

Ultimately, controllers contacted two other Northwest planes, asking them to try to reach Flight 188 through its last known frequency. One of those planes succeeded, prompting the pilot to contact Minneapolis, Church said.

“It was pretty good ATC [air traffic control] detective work,” he added.

An NTSB spokesman said the agency is examining all possible explanations for the incident, including whether the pilots might have fallen asleep.

The safety board said it is scheduling an interview with the crew and has secured the plane’s cockpit voice recorder and flight data recorder for examination. The recorders capture cockpit conversations and other noises.

Reported instances of two pilots falling asleep are rare. In August, the safety board concluded its investigation into a February 13, 2008, incident in which two pilots aboard a Go airlines flight fell asleep and traveled 26 miles beyond the destination of Hilo, Hawaii, before waking and contacting air traffic controllers.

Northwest Airlines is part of Delta Air Lines, which issued a statement Thursday, saying it is “cooperating with the FAA and NTSB in their investigation, as well as conducting our own internal investigation. The pilots have been relieved from active flying pending the completion of these investigations.”

It said Flight 188 landed safely in Minneapolis just after 9 p.m.

Delta suffered another major embarrassment this week when a Delta pilot landed a passenger jet on a taxiway at Atlanta-Hartsfield International Airport instead of the runway. The NTSB is investigating that case as well.

(Source: CNN/ Eli Rowe / YWN-008 / YWN-032)