Airlines were warned nine years ago of cracking problems in Boeing 727 wheel wells, Boeing Co. said as authorities investigated a 15-inch crack that forced an American Airlines 727 to make an unscheduled landing this week.

The bulletin "was issued because of reports from operators that they had seen cracks in those areas," Tom Cole, a spokesman for Boeing in Seattle, said Thursday. "The bulletin alerted other airlines that other operators have seen cracks in that area and that they should check."The bulletin was issued in 1979 and revised in 1981, Cole said.

American Airlines Flight 984 lost cabin pressure Monday en route from Chicago to Philadelphia and made an unscheduled landing at Detroit Metropolitan Airport. The 105 passengers and seven crew members were uninjured.

American spokesman Steve McGregor said in Fort Worth, Texas, that a 15-inch crack in the right wheel well caused the loss of cabin pressure.

The cracked portion of the airplane had been sent to the National Transportation Safety Board's laboratory in Washington, where investigators would "try to get as much information as possible about the crack," said Alan Pollock, spokesman for the agency.

The incident happened less than two weeks after a 19-year-old Boeing 737 flown by Aloha Airlines lost part of its upper fuselage over Hawaii. A flight attendant was killed, and 61 passengers and crew members were injured.

American said the two occurrences were unconnected.

Both the NTSB and American Airlines declined to speculate on what might have caused the crack. Clarke said it was likely caused by metal fatigue, a weakening of the metal due to vibration.

Cracking has become a major concern in the airline industry with the aging of the U.S. commercial air fleet, Clarke said.