WASHINGTON — Four U.S. airlines are under investigation and face possible fines for failing to comply with federal aviation regulations.

Federal Aviation Administration officials say an audit of airline maintenance found three of the airlines had missed inspection deadlines. At a briefing at Washington's Reagan National Airport, the officials said that wiring in the wheel well was one area where carriers failed to comply with "air worthiness directives."

The agency would not name the carriers under investigation. FAA officials said penalties could be levied, though it would be several months before the probe was complete.

The report comes after several weeks of controversy over the FAA and airlines' safety inspections. Major U.S. carriers, including United Airlines today, have grounded planes and canceled hundreds of flights, to follow up on inspections. American and Delta airlines canceled about 700 flights last week in order to inspect wiring in Boeing MD-80 planes.

The FAA imposed a record-breaking fine of $10.2 million against Southwest Airlines last month for flying planes despite lapses in safety checks.