The piece from a Boeing 767's wing flap was found last week wedged in an 18-inch space between two buildings in lower Manhattan.
Now officials are sifting the soil in that space to determine if any human remains also went unnoticed, CNN reported.
"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" the 5-foot-long, 4-foot-wide and 17-inch-deep piece went unnoticed, police 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," he added.
Some family members of victims of the terror attacks said they're outraged officials didn't find the debris sooner and wonder how much is still out there, CNN reported.
"I'm disgusted, because after 9/11, the proper search was never done. The whole aftermath was uncoordinated. It was inadequate," said Sally Regenhard, whose son Christian, a probationary firefighter, died when the towers collapsed.
"We advocated for a huge trajectory from that collapse," she said. "Human remains were thrown at least for a mile, but probably 2 or 3 miles from the site."