An aircraft part from one of the jets that struck the World Trade Center and was found in an alley near Ground Zero -- initially believed to be part of the landing gear -- has been identified as part of a wing flap control system, officials said.
The NYPD also clarified why a piece of rope was found on the flap support. When police first responded to reports of the wreckage last week they used the rope to move it to look for a serial number or other identifiers, officials said.
The NYPD Monday continued to guard the site, which is two blocks north of Ground Zero.
Officials with the city medical examiner's office were testing the tight space where the part was found for possible toxins ahead of Tuesday's search for possible human remains.
Officials of the Boeing Company, which manufactured both of the aircraft that terrorists used to destroy the towers, said the debris found on Friday was part of a flap actuation support structure from a Boeing 767.
However, since the planes, which struck the north and south towers, were both Boeing 767s, it was impossible for officials to say for certain from which plane the 5-foot-tall piece came from. Videos of the south tower being hit during the attack showed a fiery explosion and an ejection of debris that appeared to go a few blocks north of Ground Zero.
Ellen Borakove, a spokeswoman for the medical examiner, said sifting of the soil in the narrow alley was scheduled to begin Tuesday. She said technicians and forensic experts would use a fine gauge screen to sift material and soil at the site, which is behind a building at 51 Park Place, and at one time the site of an Islamic center.
Any possibly human remains that are found would be taken to the Manhattan morgue building at 520 First Ave., where forensic anthropologists would examine them. If the materials are deemed to be human, experts would then see if viable DNA samples could be extracted, Borakove said.
The medical examiner's office is already involved in sifting nearly 60 truckloads of World Trade Center debris and has recovered more than 60 pieces of potential human remains. But anything found behind 51 Park Place will be handled separately, Borakove said.
Once sifting is completed, possibly by Wednesday, the NYPD's emergency services unit will remove the flap support and keep it at the department's property storage facility, said spokesman Paul Browne.
Police said the debris might ultimately be given to the National Transportation Safety Board, which has treated Sept. 11 aircraft parts as historical artifacts.
With Maria Alvarez
The story has been changed to reflect that the National Transportation Safety Board does not distribute pieces of Sept. 11 artifacts as historical artifacts to museums.