New York (CNN) -- New York City officials on Monday were preparing the site where a part believed to be from one of the 9/11 airliners was found so that they can begin the search for human remains, according to the New York Police Department.
Staff from the Office of the Chief Medical Examiner will start sifting through soil for human remains on Tuesday, police said.
A Boeing Company technician confirmed that the piece, discovered Wednesday, is a support structure from a trailing edge flap found on the wing of a Boeing 767 like the ones that were flown into the World Trade Center on September 11, 2001.
It could not be determined from which of the two planes the part came, police said Monday.
The piece was found in an 18-inch gap between two buildings near ground zero. One of the buildings is 51 Park Place, the site of a controversial Islamic community center.
Surveyors who found the piece called authorities to report that they'd found "apparently damaged machinery," police said.
Contrary to an initial theory that the piece could have been lowered by a rope found still wrapped to it, the police department clarified Monday that a police officer had attached the rope last week to maneuver the part to better see a serial number and other indicators.
The plane part is about 5 feet long, 4 feet wide, and 17 inches deep, police Commissioner Ray Kelly said.
Investigators are still trying to determine how the support structure became wedged into such a small space.
"If you see how confined this space is, and you realize the chaos that existed on this street, I think it's understandable. It's not that surprising," Kelly said.
"It's very, very confined, and no construction work went on, or no clean up went on in this 18-inch space between the two buildings" after the attacks.
Some family members of 9/11 victims are outraged by the discovery.
Sally Regenhard, whose son Christian, a probationary firefighter, died when the towers collapsed, said: "I'm disgusted, because after 9/11, the proper search was never done. The whole aftermath was uncoordinated. It was inadequate."
Regenhard has been one of the leading voices and, at times, critics for the families of the 9/11 victims, pushing hard for a federal investigation and calling for a more comprehensive search for remains and debris.
"We advocated for a huge trajectory from that collapse," she said. "Human remains were thrown at least for a mile, but probably 2 or 3 miles from the site."
In recent years, debris and human remains have been discovered in various spots around Lower Manhattan.
Police are treating the area as a crime scene and say the part of wreckage will remain there until a final home is chosen.
In the past, the National Transportation Safety Board has taken some parts, while others have become part of museums.
Sharif El-Gamal, president of Soho Properties, which owns 51 Park Place, said, "We are cooperating fully with the appropriate authorities to make sure this piece of evidence is removed with care as quickly and effectively as possible."
This is not the first time this building has had national attention. It was the center of many protests while Park 51, the Islamic center, was seeking permission to locate there in 2010. Critics said the center's proximity to ground zero was unacceptable, while supporters argued it was meant to bring people together.
CNN's Chelsea J. Carter, Rob Frehse, Chris Kokenes and Mary Snow contributed to this report.