NEW YORK • His family has his spare firefighter uniform, but not the one he wore on 9/11 — or any other trace of him.

Killed at the World Trade Center, 32-year-old Scott Kopytko's remains were never recovered — a painful legacy of grief for families looking for answers, closure or final confirmation that their loved one was actually a 9/11 victim.

"Very painful and very hurt" is how Russell Mercer, Kopytko's stepfather, describes it. "And mistrusting of everybody."

Numbers tell the story in the decade of search and recovery of the remains of Sept. 11 victims — a massive forensic investigation marked by a Supreme Court appeal of families who wanted a more thorough search, and discoveries years after the attacks of even more remains in manholes and on nearby rooftops.

Tens of millions of dollars have been spent, including on the painstaking extraction of DNA from tiny bone fragments, using technology refined from a decade ago. Of 21,000 remains that have been recovered, nearly 9,000 are unidentified, because of the degraded condition in which they were found. More than 1,100 victims have no identifiable remains.

And the pace of the process is telling — in the last five years, only 26 new identifications. Ernest James, 40, who worked in the trade center's north tower, was the last identification, in late August.

"I can't give a time frame of when an identification is going to be made, if at all," said Mark Desire, who heads the World Trade Center identification unit for the city medical examiner's office. "But we are working nonstop."

Five scientists work seven days a week trying to make new identifications at a lab in an ultramodern building on the east side of Manhattan. The unidentified remains are stored in climate-controlled conditions under a white tent blocks from the medical examiner's office. About 400 bone fragments are looked at and analyzed every month. Most of the DNA profiles belong to previously identified victims.

The struggle to identify the 9/11 dead began almost immediately after the attacks in New York, the Pentagon and in Shanksville, Pa., where one of the hijacked planes crashed in the woods and plains before reaching its intended target. Forensic teams at the three sites were faced with challenges in identifying victims and the hijackers — some of whose remains are now in the custody of the FBI.

In Pennsylvania, the heat caused by the high-speed crash into a field caused 92 percent of the human remains to vaporize, leaving very little to work with, said Wallace Miller, the county corner who helped to identify the victims. DNA was used to make matches to the 40 victims, plus four sets of remains from the terrorists. To this day, remains are still embedded in the field where the flight went down.

Most of the 184 victims at the Pentagon also were identified using DNA. All but five — where there was not enough material to analyze — were identified, said Paul Stone, spokesman for the Armed Forces Medical Examiner System.

But nowhere was the forensic detective work as demanding and daunting than at the 16-acre World Trade Center site, where the giant towers collapsed onto the rest of the complex.

Few full bodies were recovered. Then, heat, moisture, bacteria and chemicals such as jet fuel combined to thwart the detective work of forensic scientists. Some remains were so badly burned or contaminated that DNA could not be analyzed.

By April 2005, the city's chief medical examiner, Charles Hirsch, told families his office would be suspending identification efforts because it had "exhausted the limits of current DNA technology."

Identifications restarted in 2006 amid the discovery of dozens of bone fragments of a 9/11 victim in a manhole renewed a search that found nearly 2,000 new fragments on rooftops and under a trade center road. The latest search ended last year.

Body parts were also recovered at a former landfill in Staten Island, where debris from the site was transferred. In 2008, a judge rejected a lawsuit brought by several families of Sept. 11 victims who sought to move the debris to a plot of land that could be transformed into a cemetery.

What to do with the remains that have been recovered also has stirred up anguish for some families.

As an above-ground memorial to the victims nears opening, some renewed objections to a plan to store the unidentified remains behind a subterranean wall at the National September 11 Memorial & Museum. The museum is to open next year.

The memorial foundation and several family members have said they agreed to the arrangement years ago. There will also be a private room adjacent to the repository set aside for families.

The inability to identify so many victims has affected the final victims' count over the years. The city's list of the dead peaked at nearly 7,000 in the months after the attacks but dropped to 2,752 by the fall of 2002.

Twenty-seven profiles DNA generated so far don't match any of the approximately 17,000 genetic reference materials that were collected. Scientists aren't sure who they are.