US Airways has found and fixed faulty wing fasteners on additional B-757 jetliners since part of the wing on one of its 757s broke off and struck a passenger window somewhere over Maryland last month, federal investigators said Tuesday.

In a written interim report on its investigation, the National Transportation Safety Board said that a 4-foot-by-5-foot panel of the wing flew off Flight 1250 on March 22 because the three clips that secured the leading edge of the panel to the wing had failed -- two before the flight because of metal fatigue and the third during the flight.

The NTSB said it was still investigating to determine whether the wing clip failure was caused by poor design, improper installation, or lax inspection and maintenance. The safety board also is still studying how its factual findings might affect the entire 757 fleet.

The safety board did say that the clips that failed were redesigned fasteners installed in 1991 after problems with the 757 wings were identified in the late 1980s.

US Airways reported to the safety board that it had inspected all its 757s since then and found "problems with wing fasteners on several other aircraft, which were since repaired and returned to service," the report said. US Airways inspected all 18 of the 757s it owns that use these fasteners and found cracked wing fasteners on six of them, in addition to the plane that lost the wing panel, said NTSB spokesman Peter Knudson.

The inspections of US Airways 757s occurred amid a flap over lax enforcement of safety orders by the Federal Aviation Administration. In the most disruptive episodes, Southwest Airlines and American Airlines canceled flights to re-inspect or repair aircraft that the FAA had ordered them to look at much earlier, but which the agency had allowed the airlines to continue flying either without doing the required inspection and maintenance or without doing it correctly.

The NTSB said it had still not found the 20-square-foot panel from Flight 1250. No one was hurt, but the outer pane on one passenger window was cracked by the errant wing panel. The inner pane was not damaged, and the aircraft remained pressurized during the flight.

Flight 1250 was carrying 174 passengers and six crew members from Orlando, Fla., to Philadelphia at the time, and it landed safely in Philadelphia about 30 minutes after the wing panel broke loose.

The safety board said analysis of information from the flight data recorder and interviews of Flight 1250 crew members showed there was no substantial change in the aircraft's handling characteristics after the panel flew off, so the safety board reclassified the event from an accident to an incident.