New York Police Department (NYPD) confirmed that the five-foot-long part was discovered by building surveyors between two buildings on the site where a mosque and community center has been proposed three streets away from Ground Zero.
The part bears a "Boeing" stamp, followed by a series of numbers.
Police spokesman Paul Browne said the NYPD had secured the location and would treat it as a crime scene.
Here's time lapse footage of New York City's One World Trade Center being built. It's now the city's tallest building.