Forensic experts have ended efforts to identify the remains of nearly half of those killed in the World Trade Center attacks of September 2001.
More than 2,700 people died in the attacks on New York
New York's Medical Examiner's office has so far identified the remains of 1,585 of the 2,749 victims who died.
But of the almost 20,000 specimens retrieved from the site, some 10,000 have yielded only partial or no DNA.
The remains will be kept in a memorial amid hopes that future forensic science will make identification possible.
The Medical Examiner's Office says it will notify the families of the unidentified victims.
The city has about 10,000 unidentified bone and tissue fragments that cannot be matched to the list of the dead.
Spokeswoman Ellen Borakove said the department would still have access to the remains, but that it had exhausted all its current options.
"We are hopeful in the future of getting new technology whereby we'll be able to identify those remains," she said, the AFP news agency reports.
Sue Rosenblum, whose 28-year-old son, Joshua, died in the attacks on the World Trade Centre, said the news came as a blow.
"It's really hard, not having anything," she told AFP.
"Time doesn't make the pain go away," she added. "It just makes the edges a little less rough."