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Plane wreckage found last week behind a building in Lower Manhattan and apparently deposited there in the Sept. 11 attacks is part of a wing flap — not part of the landing gear — from a jumbo jet of the same model as those that crashed into the World Trade Center, the Police Department’s chief spokesman said on Monday.

The police had initially described the damaged machinery, wedged into a narrow alley off 51 Park Place, as a piece of the landing gear of a Boeing 767.

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But a technician from Boeing told detectives on Sunday that the part was in fact the support structure for a mechanism connected to one wing’s trailing edge flap.

“It is believed to be from one of the two aircraft destroyed on Sept. 11, 2001,” the spokesman, Paul J. Browne, said in a statement, “but it could not be determined which one.”

A spokeswoman for Boeing explained the function of the part, which controls the motion of flaps on the wing. “When trailing edge flaps are extended, they allow the airplane to fly at slower speeds,” the spokeswoman, Kate Bergman, said. “The movement of the trailing edge flaps is often visible from passenger windows during takeoff and landing.”

An explanation also emerged on Monday for what had been among the more mysterious aspects of the find last week: a piece of rope that was entangled in the plane machinery, but not part of it. Its presence led Police Commissioner Raymond W. Kelly to raise the possibility that the wreckage had been placed at the location – perhaps lowered down from above – and fueled all manner of speculation online.

But Mr. Browne said that follow-up interviews with those who first responded to the site revealed that it was a police officer – a member of the Emergency Service Unit – who lashed a bit of rope found in the alley around the wreckage “in order to move it in such a way as to look for its serial number or other identifiers.”

The police kept the area cordoned off as a crime scene on Monday, with a mobile command truck from the Office of the Chief Medical Examiner parked out front.

Ellen Borakove, a spokeswoman for the medical examiner’s office, said a process of sifting the area around the plane part for human remains would begin on Tuesday morning.

After that is completed, the police said, the part would be kept by the Police Department’s property clerk until a decision is reached on where it should be permanently housed.