Workers discovered what was initially thought to be landing gear on Wednesday between a luxury loft rental building and a mosque that in 2010 prompted virulent national debate about Islam and freedom of speech because it's just blocks from ground zero.
On Saturday, yellow police tape blocked access to a metal door that leads to the hidden alley behind the planned Islamic community center, known as Park51.
The chief medical examiner's spokeswoman, Ellen Borakove, said the area first will be tested as part of a standard health and safety evaluation for possible toxicity.
Retired fire department Deputy Chief Jim Riches, who lost his son in the terrorist attacks, visited the site on Saturday. He said the latest news left him feeling "upset."
"The finding of this," he said, "just goes to show that we need federal people in here to do a comprehensive, full search of lower Manhattan to make sure that we don't get any more surprises," as happened in 2007 when body parts were discovered in nearby sewers and manhole covers.
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.
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.
Detectives also solved a mystery about a piece of rope intertwined with the part. A responding officer on Wednesday used rope that he found on the ground nearby to wrap around the aircraft part in order to move it in such a way as to look for its serial number or other identifiers.
Police said the part would be moved to a more secure location likely later this week, where a determination will be made about where it will go permanently.
On Tuesday, workers will begin sifting through debris in the alleyway to look for human remains.
(The Associated Press contributed to this report.)