WASHINGTON – FBI investigators found a stout “fighting” knife among the wreckage at the United Airlines Flight 93 crash site in western Pennsylvania, says a federal official who reviewed photographs of evidence at FBI headquarters here.

It’s no secret that Islamic terrorists carried knives on board Flight 93 and the three other planes they hijacked on Sept. 11, but they were widely thought to be box cutters and possibly other short-bladed knives.

Any knife with a blade less than four inches, including box cutters, was allowed on planes under Federal Aviation Administration rules before the attacks.

“One of the knives found at the crash site in Pennsylvania was one of those belt-clip, serrated, locking-blade knives,” the official said. “Its design is purely and simply a fighting knife.”

He added: “Knives such as these should not have been allowed through (airport) checkpoints,” assuming the knife was not stowed in checked baggage.

Flight 93 took off from Newark, N.J.

The official, a specialist in aviation security, says he saw a large photograph of the so-called fighting knife, broken in “two or three pieces,” hanging on the wall, along with other photos of evidence collected from crash sites that day, at the FBI’s Strategic Information and Operations Center. SIOC [SY-OCK], as it’s called, is where the FBI stages major crime investigations.

Hijacking ringleader Mohammed Atta bought two knives before the attacks – one on June 25, 2001, in Dubai, U.A.E., and another on July 8, 2001, in Zurich, Switzerland, according to the Justice Department’s indictment of suspected “20th hijacker” Zacarias Moussaoui.

The FBI declined to comment on the Flight 93 knife.

“That would be considered evidence in an active investigation, so we can’t comment,” said FBI spokesman Paul Bresson.

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