right hand, a forearm and a clavicle, and the DNA they carried, were all investigators had to identify the remains of Timothy Stout, who worked on the 103rd floor of the north tower of the World Trade Center.

Two fingerprints and a dental pattern proved key to confirming the death of David Suarez, who worked a few floors below.

A genetic analysis of a bone fragment determined the final fate of John C. Hartz, who was on the phone with his wife describing the horror of the first attack when the south tower, where he worked, was struck by a second hijacked plane. "I have never been able to understand why people have been so intent on recovering bodies," said Mr. Hartz's widow, Ellie. "Now I understand. It is a basic human need. We are tactile."

These confirmations, achieved in the last month, are each scientific miracles made possible by the largest forensic investigation in United States history, one that is pressing the limits of biomedical research even as it brings a painful mixture of relief and fresh grieving to families. But these are just 3 out of 972 identifications that investigators have made as of Friday.

A third of the 2,824 victims of the World Trade Center attack have now been identified, a number far beyond what many had thought would be possible. The goal now, experts involved in the effort say, is to use new scientific techniques to identify half or even two-thirds of the victims, despite the miserably deteriorated state of many of the remains being pulled from ground zero.

The endeavor spans the nation, from genetics laboratories in Utah, Texas, Maryland and Virginia to law enforcement bureaus in Washington and Albany; even a California forensic statistician is helping. But the federally financed job, of course, is centered in New York City, at the World Trade Center site, where remains have been meticulously collected, and at the medical examiner's office, at 520 First Avenue in Manhattan, where 18 refrigerated trailers hold the evidence.

To date, 18,937 body parts have been recovered, along with 287 whole bodies. Most of the first successes in identifying victims have come through traditional resources like fingerprints and dental records, and those techniques are still yielding results. But because of the extraordinary trauma involved in the towers' collapse, DNA is often the only hope of matching remains to a name, a family, a life story. In fact, through Friday, only 10 victims so far have been identified solely by visual confirmation.

DNA, first used as a forensic tool in 1985, led to the identification of all of the bodies in a Swissair plane crash in 1998 and an EgyptAir plane crash in 1999, two accidents in which jets plunged into the Atlantic. In the days after the Sept. 11 attack, city officials announced that they felt compelled to test each bit of human remains that could be found.

"This is an historic event of unprecedented magnitude, and the question was if the scientific community could respond to that need," said Mark D. Stolorow, executive director of Orchid Cellmark, a genetics company. "The response has been surprisingly swift. We are scientists, but we are also American scientists."

Progress has not come at an even pace. Only 2 of the 65 people aboard United Airlines Flight 175, which struck the south tower, have been identified, according to city records. By comparison, 182 of the 343 city firefighters, who wore protective gear, have been identified.

Since the day of the attack, the identification effort has proceeded simultaneously on multiple tracks. Dental records, details on any tattoos, engraved rings or other unique items were collected by the police, in the hope that traditional identification approaches might be sufficient. But city investigators also started immediately to assemble DNA from victims' families, who supplied toothbrushes, razors, even lip balm used by a victim, which presumably would contain his or her DNA. Cheek swabs from the victims' relatives were also taken.

Each person's DNA, or genetic code, consists of a string of three billion "base pairs," or large molecules, represented by the letters "A," "G," "C," and "T." Sequences of those four molecules create the code for all human characteristics, and variations in those sequences make one person different from another. Those same variations also allow DNA to be used like a fingerprint.

To start this effort, the city relied on a well-proven DNA technique, called Short Tandem Repeat, in which the laboratories looked for 13 different markers in each sample of human remains collected from ground zero, measuring the size of each marker and assigning the equivalent of a Social Security number to each fragment of remains. An analysis would also be done on the 6,908 razor blades, combs, toothbrushes and other personal items, and the 6,889 cheek swabs from victims' relatives.

Myriad Genetics of Salt Lake City and the Bode Technology Group of Springfield, Va., handled most of this initial work. Bode alone has been sent 12,000 bone samples, 5,500 soft tissue samples and 1,800 samples from family members. The results are being sent back to the New York State Police, and then the city medical examiner's office, where staff members start on the difficult work of matching DNA profiles from the remains with those from the family items and confirming the accuracy of each step.

This effort gradually started to produce significant results: 57 DNA identifications in November, 69 in March and 92 in April, as of Friday. But nothing is coming easily.

The fires that burned for weeks after the towers fell were so hot that even when bones were recovered, they were often little more than ash. The moisture at the site and bacteria caused further degradation. The result is that nearly half of the first round of samples tested at DNA labs have come back with incomplete profiles, city officials said.

In as many as 700 cases, the medical examiner's office has been unable to link a DNA profile that was isolated from a piece of remains with any of the profiles established based on the items supplied by the victim's families. Making the matches has become almost an obsession for Dr. Robert Shaler, the director of forensic biology with the city's medical examiner's office. He finds himself at his office at 5 a.m., at his computer, again and again, trying to make just one more match. He wonders as he arrives for work: "Can I make matches? Can I make matches?"

Former Mayor Rudolph W. Giuliani said he already was amazed at the success Dr. Shaler, and his boss, Dr. Charles S. Hirsch, the city's chief medical examiner, have had. "I honestly think on the evening of Sept. 11th, none of us who observed it, saw it, watched it, were involved in it, ever thought you would have been able to identify a third of the people," Mr. Giuliani said.

But Dr. Shaler and other city officials say they are far from satisfied. They believe they have another eight months of work, as they are just now pushing ahead again, in a second wave of testing.

Celera Genomics, a Maryland company best known for its work in sequencing the human genome in recent years, is applying its fast DNA sequencing machines to the World Trade Center identification effort. Celera's work, in conjunction with its Applied Biosystems division, is focusing on tiny rings of DNA in cell structures called mitochondria. These maternally inherited rings are hardier than the long strands of DNA used in the more traditional tests, and there are as many as 10,000 of them in each cell, giving investigators much more to work with. This approach has been used before — including the 1994 identification of the remains of Czar Nicholas II of Russia — but never before on such a large scale.

The city is also turning to techniques that have never been used before in forensic investigations: single nucleotide polymorphisms, known as snips, are telltale variations in single base pairs scattered throughout the genome — an A instead of a T, say. The snips can be found even when a victim's DNA has been broken into fragments as short as 60 to 80 base pairs, much less than required in the traditional tests, Mr. Stolorow of Orchid Cellmark said.

In preliminary attempts, the success rate for developing DNA profiles of victims who could not be identified with the other methods has been "encouragingly high," Mr. Stolorow said. The full process of getting profiles, matching them with DNA from relatives and other sources is expected to take two to three months, he added.

These incursions into uncharted scientific territory and even the identifications that have come from traditional means have produced a volatile amalgam of deep gratitude, a resurgence of September's searing grief, the need to grapple with unfamiliar choices, and more than a few surprises in the worlds of bereaved families.

One surprise lay hidden in the hopes of 12-year-old Brendan Regan until the remains of his father, Robert Regan, a lieutenant in Engine Company 205, Ladder Company 118 in Brooklyn Heights, were found and identified on New Year's Day. The results came quickly, based on dental records and a medal of St. Florian, patron saint of firefighters, inscribed with his children's names.

"It turned out that up until that point, my son had held out an unbelievable hope in his heart that he was still capable of having a miracle occur," Lieutenant Regan's widow, Donna Regan, said. "He felt my husband may have crawled to a safe spot" and somehow survived, she said. Now, Mrs. Regan hopes, her son can begin the long and difficult process of healing.

But the prospect that science could again and again identify more of a victim's remains has put some families in a torturous limbo. "We decided to hold off on the funeral," said Robert Alonso, whose wife, Janet Alonso, worked for Marsh & McLennan on the 95th floor of the north tower. Some of her remains were identified less than two weeks ago.

"The last thing we needed was to have a service and then say, `They've found more remains at ground zero,' " Mr. Alonso said.

The impact on families of techniques that can identify almost any fragment of a loved one's remains is not always positive. "It's very upsetting," one widow said of the news. "I almost threw up."

Given those emotions and the fact that dozens of distinct remains are being found at times from a single victim, the medical examiner's office is giving families the option of being notified only once, when the first confirmation is made. They are also giving families the alternative of leaving any identified remains at the morgue until all testing is over, so that a single burial can take place.

Still, everyone expresses thanks for the monumental effort taking place at ground zero and at labs across the country. The identifications help families escape what Mrs. Regan calls the "vanish factor": not having anything tangible on which to focus the last goodbyes.

Mr. Alonso said thoughts of his children, ages 2 and 3, help him cope with the upwelling of grief that the identification of his wife has brought. "Questions will be coming as we get older: `Where's Mommy? What happened to Mommy?' " Mr. Alonso said. A grave site, he said, "brings me a place where when the kids get older and understand, I can bring them and show them something."