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Veteran aviation accident investigators say they have never seen a scenario like the one that played out in the final moments of American Airlines Flight 587. They have little doubt about what led the jet to crash Monday in New York City: Large pieces came loose shortly after it took off. But that hardly solves the mystery. Investigators are still searching for what could have caused the Airbus A300 jet to break into pieces and slam into homes in the Rockaway Beach neighborhood of Queens about 9:17 a.m.

The two General Electric CF6 engines landed separately in the neighborhood, each several blocks away from the fiery main crash site, federal sources say. The Coast Guard also pulled a part of the A300's tail from Jamaica Bay.

"Right now, to me, it is extremely mysterious. I find the evidence perplexing," says Bernard Loeb, the retired chief aviation investigator with the National Transportation Safety Board.

Engine failures, while rare, occur dozens of times each year. In extraordinary circumstances, engines fail so violently that they shoot damaging shrapnel into an aircraft.

However, it's unheard of that two engines would break loose at or about the same time. Jet and engine manufacturers go to great lengths to ensure that catastrophic engine failures cannot cause a crash.

Similarly, jets are designed with so much strength that even when the wings or tail fins are damaged, they almost never break loose before crashing, investigators say.

The part of the tail — a fin that rises vertically — that was recovered had no visible marks indicating it was struck by an object that could have torn it from the jet.

Even the early evidence about how the jet broke apart puzzled investigators and others. Some pilots who saw the jet after takeoff said it did not come apart, while other aviators said they saw pieces coming from it, one source says.

"It doesn't sound like the typical thing," says Kevin Darcy, the former lead accident investigator at Boeing who is now an aviation consultant.

Loeb says it is possible that the key to the crash could be with the tail section, which was hauled from the bay Monday afternoon. The vertical fin keeps the jet pointed straight and allows pilots to turn the jet's nose left or right.

If the small wings on the tail, which raise and lower the nose, also came loose, Loeb says, that could cause the jet's nose to move downward with great force. That, in turn, could theoretically shake the engines off of the wings.

Loeb cautions that such a scenario is highly speculative. "I don't really know exactly. But I think that's possible," he says.

Whenever engines break off a jet, investigators focus on possible failures within the engine. The CF6 engines on American's A300 fleet have recently drawn the attention of safety regulators.

The NTSB issued a recommendation last December urging that the Federal Aviation Administration, which regulates the aviation industry, address safety issues on the CF6 engines.

On Sept. 22, 2000, a CF6 engine on a US Airways Boeing 767 blew up as mechanics tested it on the ground in Philadelphia. There was a loud explosion, and a fire broke out under the left wing of the jet, the NTSB reported.

"The incident raises serious safety concerns because, if it had occurred during flight rather than on the ground during maintenance, the airplane might not have been able to maintain safe flight," the NTSB wrote.

Other incidents involving the engine have also raised concerns among investigators.

In April 2000, a Continental DC-10, which had three CF6 engines, had one engine break apart as it took off from Newark International Airport. Pieces from that engine damaged a second engine. The crew landed the jet after burning off fuel, and no one was injured.

Since then, the FAA has required additional inspections of all CF6 engines. "My understanding is that the airplane was in compliance with all airworthiness directives," says Al Becker, spokesman for American.

Birds are another possible cause of engine failure. The marshy area near New York's Kennedy International Airport, where Flight 587 took off 3 minutes before the crash, is a haven for birds. Several crashes, though not on A300 jets with GE engines, have been caused by engines exploding after birds were sucked into them.

However, several accident investigators say it seems unlikely that such failures struck Flight 587. Major failures on jet engines are often obvious after a crash, but sources said the engines exhibited little evidence of such a failure. The sources cautioned that such on-scene observations are not always reliable.

Furthermore, earlier failures of the CF6 engines have not caused them to break off. On the US Airways 767, for example, the engine remained on the wing.

In fact, one engine specialist who asked not to be identified, said modern, high-power engines like the CF6 have failed so rarely in flight that investigators have little idea what would happen to a jet if one came apart.

Investigators also say it seems unlikely that failures violent enough to shake each engine loose from the aircraft could strike both at about the same time.

Though federal officials went out of their way to say no evidence suggested sabotage, officials say privately that they cannot rule it out. It is possible, safety experts say, that a bomb or a missile downed the jet. In the past, terrorists have attempted to shoot down jets using small handheld missiles. Loeb says an explosion at the rear of the jet could have caused the tail section to break free.

Investigators hope the answers will come soon. The voice and data recorders, which have been recovered, will offer important evidence about how systems aboard the jet were functioning and what the pilots were saying in the final moments.