Of the nearly 3,000 victims, Riches noted, about 1,000 families have never recovered any remains.
The New York Police Department has declared the alley a crime scene where nothing may be disturbed until the medical examiner's office completes its work. It's unclear how long that may take, Borakove said.
The piece of wreckage was discovered by surveyors inspecting the planned Islamic community center on behalf of the building's owner, police said.
The twisted metal part -- jammed in an 18-inch-wide, trash-laden passageway between the buildings -- has cables and levers on it and is about 5 feet high, 17 inches wide and 4 feet long, police Commissioner Raymond Kelly said Friday."It's a manifestation of a horrific terrorist act a block and a half away from where we stand," he said after visiting the alley. The commissioner noted that a piece of rope intertwined with the part looks like a broken pulley that may have come down from the roof of the Islamic community center. When plans for the center became public in 2010, opponents said they didn't want a mosque so close to where Islamic extremists attacked, but supporters said the center would promote harmony between Muslims and followers of other faiths. The building includes a Muslim prayer space that has been open for three years. After protests died down, the center hosted its first exhibit last year. The space remains under renovation. ___ AP radio correspondent Julie Walker and AP reporter Jennifer Peltz contributed to this report.