In the two decades since the Sept. 11 attacks, forensic scientists have been hard at work trying to identify the 2,753 people who were killed at the World Trade Center -- but the road hasn't been easy.
As of this week, the Office of Chief Medical Examiner (OCME) in New York City has identified 1,647 victims, mostly using DNA from the human remains found at the site of the attack. But still, 40% of the Ground Zero victims haven't been identified.
Mark Desire, assistant director of the OCME Department of Forensic Biology and manager of the World Trade Center DNA Identification Team, told reporters during a video call Wednesday that the investigation has been challenging, but after all this time, the team's mission remains the same: to help the families of the victims find some closure.
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"We've adapted, we've overcome and we've pushed that science, day after day," Desire said.
Desire and other forensic scientists who've worked on the identification project said there are numerous factors that have made identification difficult. For starters, the remains at Ground Zero were exposed to several elements that can destroy DNA, including jet fuel, mold and fire.
John Butler, a fellow at the National Institute of Standards and Technology, who assisted with the World Trade Center identification efforts shortly after the attacks, told ABC News that it has taken a long time to get a manifest of all of the fatalities at the site, which included first responders, airplane passengers and crew, office workers and other civilians.
He noted that scientists had to gather as many DNA samples from the victims -- whether it was from old toothbrushes, pieces of clothing or close family members -- as possible to have a database to match the remnants found at Ground Zero.
"There wasn't an inventory of everyone in the lab. They had to go out and get samples," he said.
Desire also said there are victims for whom no additional DNA samples have been able to be retrieved.
"Maybe [the victim didn't] have family or the family has accepted it and don't want to be notified," he said.
Over 22,000 human bone and tissue samples have been recovered from Ground Zero over the years, according to OCME. And by the end of May 2002, 736 victims were identified without DNA matching, according to a report issued by Butler a decade ago.
But he said one of the biggest issues that forensic experts still struggle with is the fragmented nature of the DNA recovered from the site, which is why he worked for months to come up with a new type of analysis for short tandem repeat (STR) markers in the DNA, which are unique among related persons.
"You can recover more information from a sample that has been broken into smaller pieces," Butler said, adding that 20% of the identifications that have been made so far were done using the "miniSTR" tests that he helped develop.
Desire said the World Trade Center DNA Identification Team continues to work on analyzing the data they've gathered from the victims and making those DNA matches. On many occasions, the tests on a sample will match with a victim who has already been identified, but those matches still help in the long run, because it gives the team a more defined blueprint of that victim's identity.
"It's the samples we have gone back to over and over and over again," he said.
Desire said his team is in constant contact with victims' families -- many who are still awaiting for their loved ones to be properly identified.
On Tuesday, for the first time in nearly two years, the office identified two victims. One of them is Dorothy Morgan, of Hempstead, New York, who worked for an insurance company at the North Tower. The identity of the second match is being withheld from the public at the family members' request, OCME said.
Nikiah Morgan, Dorothy Morgan's daughter, told WABC that she held out hope for years that "she was just out there somewhere."
"I didn't expect it after all this time," she said.
Desire said he can't give a timetable of when the next identifications will be made, but he believes new technology will lead to more matches.
The World Trade Center DNA Identification Team will soon be using a process known as next generation sequencing, which has been used by the military for long-term investigations involving unknown victims.
MORE: More people believed to have died from 9/11-related illnesses than in the attacksDesire said the technique has helped identify the remains of soldiers who died as far back as the Korean War.
"It allows us to look at samples that we had no hope in the past," he said.
Overall, Desire said his team's progress over the last 20 years has been remarkable, given the scope and conditions of the investigation. However, he said they're still pushing to complete the identifications, for the sake of the victims' loved ones.
"We won't stop, because we know we can continue to make identifications," Desire said.