With 40 percent of the victims of the Sept. 11 attack on the World Trade Center still unidentified, experts are holding out hope that improvements in DNA-analysis methods will allow future identification of at least some of the remains of 1,112 victims still unaccounted for.

After a flurry of identification in the first years after the attack 16 years ago — mostly through DNA but also by checking of dental records and personal effects — the process of associating names to the fragmented remains of the total 2,753 people who died at the Twin Towers has slowed to less than a trickle in recent years. Last month DNA was used to identify one female victim.

A key aspect of the problem is that the unidentified remains — totaling 7,556 fragments out of an original nearly 22,000 — were subjected to catastrophic conditions in the attacks and the immediate aftermath. That had the effect of degrading whatever DNA existed, in some cases making future identification nearly impossible, experts have said.

But in the years since the attack, the best hope to identify the thousands of remains held in a special repository at Ground Zero lies with advances in DNA techniques which, experts said, can be employed to find usable evidence where none could be found earlier, as with last month’s discovery.

“We are cautiously hopeful,” said Tim Kupferschmid, chief of laboratories at the Office of Chief Medical Examiner of the city. “These are the most difficult samples encountered in forensic science.”

Veronica Cano, a Criminalist I at the Office of Chief Medical Examiner of the City of New York's Department of Forensic Biology, quantifies an extracted DNA sample in a practice lab in Manhattan, Thursday, Sept. 7, 2017. The Office of the Chief Medical Examiner has recovered 21,905 remains and identified 60 percent of the victims of the 9/11 terrorist attack. Photo Credit: Charles Eckert

The extreme heat, pressures and contamination from the collapse of the Twin Towers in some cases destroyed usable DNA.

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“It wasn’t as hot as a crematorium, which completely destroys DNA . . . it was getting close to that hot” said Kupferschmid of the fires that burned for weeks after the towers collapsed.

Added to the heat was the impact of pressure from the collapse and contamination by chemicals, all which could degrade DNA further and in some cases make it unrecoverable, he noted.

But since the attack, DNA science has advanced on a number of fronts, Kupferschmid said. New commercially available DNA kits allow scientists to overcome obstacles such as chemical contamination and locate usable genetic material on more locations of a chromosome. The greater the number of chromosome locations, the greater the chance of making an identification, he said.

The work is time-consuming.

The first step in the process is to grind a piece of bone fragment into a powder so that it can be demineralized, leaving only biological material behind that might contain DNA, Kupferschmid explained. Then, any DNA that remains can be subjected to analysis through the new testing kit, he added.

It takes two days to treat the remains and another four days to run the analysis to get a DNA profile. If there is a reference sample, it may be possible to make an identification, Kupferschmid explained.

There are thousands of samples to be checked with the new testing kit and the process has been underway since January.

“We have done a couple of hundred since January, so it is a slow road,” Kupferschmid said.

Michael Baden, a former New York City chief medical examiner, believes that the passage of time will make it more unlikely that there will be any significant numbers of identifications from DNA analysis.

“The ability now is getting more and more remote,” said Baden in a telephone interview.