Boeing said the piece, initially thought to have been part of the plane's landing gear, "is a trailing flap actuation support structure," New York Police Department spokesman Paul Browne said in a statement.
The discovery of the metal fragment more than a decade after the September 11, 2001, attack against the World Trade Center was an eerie reminder for New Yorkers of the day when almost 3,000 people died in the collapsing Twin Towers.
On Tuesday, forensic experts will scour the site -- a narrow gap between two buildings two blocks from the World Trade Center -- for human remains.
"We are currently completing all preparations at the site (health and safety protocols, environmental testing, etc) in advance of the start of the sifting operation that will start at 8:00 am tomorrow," Ellen Borakove, spokeswoman for the New York City medical examiner's office, said in a statement.
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Al-Qaeda suicide squads hijacked two airliners on 9/11 and flew them into what were New York's tallest skyscrapers and a symbol of US financial clout. Another hijacked airliner was flown into the Pentagon building outside Washington, and a fourth crashed into a Pennsylvania field.
The search for bone or other human remains at the site is part of a much bigger, extremely slow process of trying to find fragments of victims across the zone known as Ground Zero, where a new World Trade Center is being constructed.
Body parts have never been recovered for as many as 1,122 of the people known to have died in the Twin Towers inferno, according to the Office of the Chief Medical Examiner.