Ten bone fragments discovered more than four years after the World Trade Center (search) attacks on the rooftop of a nearby skyscraper are human, officials said Friday.

The pieces were found last month by construction workers sifting through gravel on top of the former Deutsche Bank (search) building, which is being torn down.

Parts of the trade center's south tower spread debris and carved a gash in the 41-story building's facade when it collapsed on Sept. 11, 2001 (search).

The city medical examiner will try to identify the remains by extracting DNA and checking it against a database of World Trade Center victims, said medical examiner's spokeswoman Ellen Borakove.

The city has recovered 19,964 pieces of human remains from the attack and identified 9,100 of them, Borakove said. Of the 2,749 people who died at the trade center, 1,152 have no identifiable remains.

Mary Fetchet, whose son died at the south tower, said Friday the discovery reopens old wounds.

"It's just a reminder of the horrific nature of the event," she said. "When you talk about identification of bone fragments, it just brings you back to what really happened that day."