FAA spokeswoman Laura Brown said Saturday her agency had sent "letters of investigation" to Capt. Timothy Cheney and First Officer Richard Cole, alerting them the incident may quickly lead to revocation or suspensions of their pilots' licenses, The New York Times reported.
Cole has denied he was sleeping as Northwest Flight 188 flew 150 miles past its Minneapolis-St. Paul International Airport destination Wednesday evening. The Times said efforts by air traffic controllers to reach the pilots -- by radio, e-mail, data text and cell phone -- went unanswered for 90 minutes.
Officials said a flight attendant finally alerted the pilots, who turned the plane around and landed it without incident.
Cole and Cheney had told air traffic controllers they had lost track of where they were because they were arguing over airline policies. Cole later told The (Portland) Oregonian, "We were not asleep, we were not having an argument or fighting."
The pilots did not return the Times' calls for comments.
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