NEW YORK (CNN) -- A piece believed to be from one of the airliners that hit the World Trade Center towers on September 11, 2001, has been found behind the site of an Islamic community center near ground zero, the New York Police Department said Friday.
Investigators are looking into how a part of what appears to be landing gear became wedged between 51 Park Place -- the site of a controversial community center -- and another building, just a few blocks from ground zero.
"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," NYPD Commissioner Ray 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.
Even so, he said investigators are looking at all possibilities, including whether the part was intentionally placed between the two buildings.
"We are also looking into a possibility it was lowered by a rope," Kelly said, adding that a piece of rope appeared to be intertwined with the part.
But Kelly said an initial survey found no marks on the walls between the two buildings consistent with such an object being lowered. He also it appears that there is a break in the rope and that it came down from the roof on top of the plane part.
National Transportation Safety Board investigators will determine whether the part is from either United Airlines Flight 175 or American Airlines Flight 11, which hit the twin towers, he said.
Police have said the part includes "a clearly visible Boeing identification number." On Saturday, the NYPD said police investigators had been in touch with Boeing, which confirmed the part is from one of its 767 aircraft. Both flights that hit the World Trade Center towers were 767s.
American Airlines and United Airlines declined to comment.
Boeing spokesman John Dern confirmed the company was asked to "examine the images to see what can be determined."
The part was discovered Wednesday by surveyors hired by a property owner. They called 911 to report that they'd found "apparently damaged machinery," police said.
"The NYPD is securing the location as it would a crime scene, documenting it photographically," the police statement said.
The plane part will not be moved until after the medical examiner's office completes a health and safety inspection.
"The office of the chief medical examiner will do an in-depth examination of the site to see if there are, in fact, human remains there," Kelly said.
New York police officers were standing guard through the weekend outside the buildings. Investigators are expected to return Monday to determine how to remove the part.
The plane part is about five feet long, four feet wide and 17 inches deep, Kelly said.
The discovery of the plane part comes more than 11 years after the two airliners slammed into the towers of the World Trade Center.
"Well, obviously, this is a manifestation of a horrific terrorist attack a block and a half away from where we stand," Kelly said. "So sure, it brings back terrible memories to anyone who was here and anyone who was involved in that event."
Sally Regenhard, whose son Christian -- a probationary firefighter -- died when the towers collapsed, said she was not surprised by the find.
"I'm disgusted, because after 9/11 the proper search was never done. The whole aftermath was uncoordinated. It was inadequate," she said.
Regenhard has been one of the leading voices and, at times, critics for the families of the September 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 two or three miles from the site."
In the years since, a number of pieces of debris as well as human remains have been discovered in Lower Manhattan.
"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," said Sharif El-Gamal, president of Soho Properties, the owner of 51 Park Place.
The building had been the site of furious protests, pitting those opposed to putting an Islamic center near ground zero and those who say it is a center meant to bring people together.