The force that knocked the tail fin from American Airlines Flight 587 was so strong that it ripped apart six large fasteners while leaving the tail itself in surprisingly good condition.
Investigators from the National Transportation Safety Board believe the broken fin played a key role in Monday's crash, which killed all 260 people on board and five more on the ground.
The airline said Wednesday that it would inspect the fins on its 34 other Airbus A300s to see if they have flaws or maintenance problems that could make them prone to break.
Separately, the Federal Aviation Administration announced it was sending a materials expert to New York to examine the wreckage of the jetliner. The agency then will decide whether further action is necessary.
"If the analysis indicates we need to mandate any inspection or corrective measures, we'll do that immediately," said FAA spokeswoman Laura Brown.
Investigators are studying the plane's radar track to determine whether the A300 was severely jostled by wake turbulence from a Japan Airlines Boeing 747 that left the same runway at John F. Kennedy International Airport less than two minutes earlier.
The planes were at least 4 miles apart at all times, which would be in compliance with FAA requirements, but weather conditions were ideal for the powerful wakes to drift into the path of the American Airlines jetliner.
A wake vortex, a spinning tube of air that trails a big plane, is probably not strong enough to flip the 177-foot long A300 out of the sky. But it could start a chain of events that led to a crash.
For now, the key question is what snapped off the tail fin.
The best clues were visible Wednesday on the wreckage of the fuselage, where six V-shaped fasteners from the fin were connected to metal brackets with thick bolts.
The fasteners are made of a composite material called carbon fiber reinforced plastic. It has been increasingly used on jetliners because it is lightweight and strong and does not corrode.
At the crash site on the Rockaway peninsula, the bolts and the metal brackets could be seen intact on the fuselage. They still held the bottom portion of the fasteners, with carbon fibers sticking straight up, as if the tail had been ripped away.
The top portions of the fasteners apparently remained attached to the tail fin.
But what would have enough force to tear off the tail?
One possibility is a sudden, extreme movement by the plane's rudder. Aviation analysts say that might have caused the plane to quickly skid to one side, causing an unusually heavy air load on the fin that snapped it off. That could have been caused by the pilot stomping on the rudder pedal.
A 1996 test involving a Boeing 737 showed a sudden rudder movement could put significant stresses on a tail fin.
Another possibility: some kind of flaw or weakness in the fasteners. Investigators have asked for inspection records of the plane to determine how the composite material has been maintained. The same plane was involved in a 1994 incident with severe turbulence that injured 47 people.
One of the six fasteners had been repaired 13 years ago because it was peeling apart. The fix, in which the layers of composite material were riveted together, complied with federal rules, officials said.
Greg Feith, a former investigator for the NTSB, said it was possible that the A300's tail fin was weakened by the turbulence incident or other flaws and then broke off when it was hit with the wake of the Japan Airlines plane.
"When you enter into that, you start to create all sorts of aerodynamic loads on the airplane," Feith said.
The cockpit tape indicates rattling noises were heard about 25 seconds after takeoff. The captain made a comment about a wake encounter and then another rattling sound was heard.
Feith said the first encounter with wake turbulence could have been "the first punch." A second encounter might have snapped off the tail.
Investigators continued to emphasize Wednesday that they have not ruled out any theory, including terrorism. But the clues are increasingly indicating that the crash was accidental.
Attorney General John Ashcroft said that about 130 FBI agents were assigned to the crash investigation, which he said was "standard operating procedure."
"We're actively investigating all possibilities, including terrorism. To date, we have not found any evidence that this was anything other than a tragic accident," Ashcroft said.
He said agents had conducted more than 200 interviews with eyewitnesses to the crash and talked with more than 70 airline and airport employees. An explosives residue team also is examining the plane for any sign that a bomb brought it down.
Airbus, the European consortium that makes the plane, is taking part in the investigation through the NTSB's "party system," which allows manufacturers, airlines and unions to take part. But Airbus spokesman Clay McConnell said there was insufficient evidence to call for inspections of other A300s.
"Airbus has been given no indication from the NTSB that inspections are necessary," he said.
_ Times staff writer Mary Jacoby contributed to this report.