NORFOLK, VA. (AP) -- The reason a twin-engine radar plane crashed off the coast of North Carolina a year ago, killing a Maine Navy man, may remain a mystery.

Navy investigators say they have been unable to determine what caused the crash which killed three men. Among them was Lt. j.g. Jerry Smith, 25, of Greenville. Thirty-year-old Lt. Cameron Hall of Natchitoches, La., and 31-year-old Lt. Ryan Betton of Collinsville, Va., were also killed.

A report released Monday indicates the E2-C Hawkeye experienced problems with a cockpit indicator during flight the preceding day. It also notes the most recent maintenance records were on board when the plane crashed. But the said there was no evidence of maintenance issues causing the crash.

The plane went down just after takeoff from the Norfolk-based aircraft carrier USS Harry S. Truman during exercises that involve taking off and landing on a carrier deck.