THE remains of as many as 1,000 people lost in the World Trade Centre attack might never be identified, according to the forensic biologist leading the monumental DNA identification project.

The city medical examiner's office has identified slightly more than half of the 2,792 people killed in the attack - only about 100 of those in the last year - as technicians struggled with DNA degraded and damaged by fire and the elements.

Robert Shaler, chief of forensic biology, had once hoped to make 2,000 identifications, but he said he no longer considers that a realistic goal.

Now, Shaler said he hopes for about 1,700 identifications - 1,800 at the outside - by the time the office exhausts available DNA matching methods within a year. City officials recently notified victims' families of the outlook.

"I think once we've done all of the testing on all of the remains using the technology we have, I think we're finished," Shaler said.

He cautioned that he does not mean the trade centre DNA effort would be closed forever, but said it could not continue until new DNA processes were developed.

"If three years from now somebody comes up with something ... that really looks like it's going to work, then we're going to be poised to go after it," he said.

Identifications were made quickly in the weeks after the September 11, 2001, attack, many based on bodies recovered mostly intact. For smaller parts, the medical examiner has had to rely on DNA matching, drawing results from shreds of bone and tissue.

The medical examiner's office has been aggressive in finding new avenues for DNA identifications, hiring companies to write software for each technique they try. Right now, Shaler is hoping an adapted version of a DNA process normally used for disease research will soon yield new identifications.

It lets technicians examine unusually short pieces of DNA, which shrinks as it degrades. The DNA in about 61 per cent of the remains recovered at the trade centre did not yield workable pieces in initial tests because it was broken down by heat, humidity and time.

Shaler said the work in the next year would involve a tedious scrutiny of the badly damaged DNA. The samples would undergo every available process, each of which yield only partial results. Those test results are then compared; for a successful match, the idea is that each test fills in some of the gaps, like puzzle pieces fitting together.

"We're marrying different pieces of information together," Shaler said. "It's just this slow trickle."

Many of the new DNA matches made this year have yielded additional remains for people already identified. The highest number of body parts matched to one person is more than 200.

In most cases, victims whose remains have not been identified have been legally declared dead by the court and their families issued death certificates based on documents and other proof they were at the trade centre or on the hijacked airplanes.

Shaler has begun looking beyond the lab to information about where people worked in the towers, where and when their co-workers' remains were found, and who in those groups have been identified. With that data, the medical examiner's office can cross-reference identified people with unidentified samples, narrowing the search to scrutinise a few DNA profiles instead of hundreds.

Shaler came up with one potential match when he was experimenting whether this technique was feasible. He then ordered software to be created that would do it for him.

"I'm still driven by the families," he said.

"When I see these people, they look at me with eyes that say, 'Did you find her yet?' But when you're only turning out a couple a week or four, five a month, it's hard."

The Associated Press