NEW YORK — Identity — the first thing each infant learns — is the first thing death erases.

Restoring the proper name to each scrap of bone and flesh found in the rubble of the Sept. 11 attacks on the World Trade Center is likely to be the largest forensic identification effort ever conducted in the United States. So crushing was the collapse of the twin 110-story towers, so caustic the chemicals unleashed and so fierce the fires that few intact bodies are being found.

Since the attacks, scores of medical examiners, dental experts, molecular biologists and pathologists at laboratories in New York, Utah and Maryland have been struggling to identify the remains.

In the end, a million fragments of human body parts — many burned or torn beyond recognition — may be found in the wreckage, several forensic experts said. Each fragment will be bagged, tagged with a bar code, entered into a computer database, and its every characteristic — down to its DNA — studied intently.

There are other teams studying the remains of those who died at the Pentagon in Virginia and in the crash of United Airlines Flight 93 near Shanksville, Pa. But the problems facing forensic experts in New York are especially daunting.

The Pentagon has military identification tags and DNA banks to assist in identifying victims. Airlines have passenger manifests. Workers at the trade center began with only an uncertain list of thousands of missing people, including residents of 60 nations, that fluctuates faster than they can recover remains.

Of the 276 confirmed dead so far, 206 have been identified.

Identifying the rest "is an enormous task," said forensic anthropologist Clyde Snow, who pioneered many of the identification techniques now being put to the task.

No one knows how long it will take, how much it will cost, or the emotional toll it will exact from the forensic experts now working in three shifts, 24 hours a day, seven days a week.

As best they can, morgue teams inventory the remains. They note any identifying detail that friends and families of the missing can provide: a spider-web tattoo on a woman from the 100th floor; a red medical-alert bracelet worn by a 6-year-old boy visiting his mother on the 107th; a woman's name tattooed on the arm of a man from the 93rd floor; the half-circle of freckles on the broad shoulder of a broker last seen on the 89th floor.

Forensic specialists have a remarkable array of tools to match fragmentary remains with a name. Since fingerprints were first introduced as evidence of identity in a 1911 burglary trial, analysts have bolstered their diagnostic skills with techniques for facial reconstruction, chemical tests, computerized dental comparisons and DNA analysis.

They can peel fingerprints from charred hands and rebuild burned faces.

They can analyze as many as 70 enzymes in blood, bone marrow and body fluids to put together a distinctive biochemical profile.

From the long bones of arms and legs, scientists can gauge someone's age, weight, sex, race and muscularity. With enough material, they can even tell if a person was right- or left-handed.

From a single strand of hair, some forensic experts can glean estimates of race, sex and other characteristics. Researchers at the National Institute of Standards and Technology are testing a way to use sophisticated techniques for chemical analysis known as gas chromatography and mass spectrometry to develop a chemical profile from hair samples that can be used like a fingerprint.

But for thousands of the missing, whose photographs are taped to Manhattan's walls and street poles, smiles may be all that survive.

Teeth could endure in the hellish heat that melted the World Trade Center's steel beams even when all other human remains were consumed. Dental enamel can withstand temperatures as high as 1,800 degrees Fahrenheit, forensic odontologists said.

Teeth long have been a mainstay for forensic pathologists.

At the New York City Medical Examiner's office, 100 dentists have been working in shifts around the clock to assemble the needed X-rays and patient notes to match against teeth recovered from the rubble. They scrutinize each tooth with the intensity of a diamond cutter.

When medical files are handed over by relatives, the identifying dental details are entered into a rapidly growing database of the dead. Teeth recovered from the ruins then can be matched quickly against the computerized files.

For many of the missing, however, only DNA — the molecule that singles out each person with a unique chemical code — holds the key to identity.

Any tissue that survived the heat and blast should harbor viable DNA.

Under the most extreme conditions, DNA can survive intact inside teeth, protected from heat, moisture and corrosion by the tough enamel, said forensic expert Tom Glass, who supervised the dental identification of the 168 people killed in the 1995 Oklahoma City bombing.

"All you need is a few cells with intact DNA."

Laboratory technicians can take even minute DNA traces and reproduce enough to create a sample that can be analyzed. They use an automated technique called a polymerase chain reaction, or PCR, that mimics nature's way of replicating DNA.

Technicians began extracting short strings of DNA from nine specific locations in each sample. The strings, known as STRs (for Short Tandem Repeats), consist of groups of four letters in the genetic code that are repeated over and over — such as CATC CATC CATC.

The number of such repeats varies from person to person at each of the nine sites. Taken together, these numerical variations form a unique identifier for an individual, rather like a bar code.

Investigators are soliciting blood samples from the victims' relatives and looking for matching repeat sequences. But this process is not as definitive. "They can extrapolate from that," said Norah Rudin, a private forensic DNA consultant based in Berkeley, Calif.

Other personal or family evidence also can be useful.

"If they saved a lock of hair, a baby tooth or a toothbrush that contains cells that are not degraded, they can get DNA from those and try to find a match," said David Betsch, a forensic DNA expert at Bryant College in Smithfield, R.I.

For anyone born after the 1960s, hospitals save a dried blood sample, known as a Guthrie card, which also can be used for identification.

"Since we have this DNA technology, I am reasonably assured that everybody (whose remains can be found) will be identified," said Dr. Donald Reay, head of the American College of Pathologists' forensic autopsy committee and former King County chief medical examiner. "It is the pure volume that will be overwhelming. I shudder to think how you will do it."

Within hours of the disaster, the New York City medical examiner's office began collecting DNA samples from tissue discovered in the wreckage.

The city has perhaps the largest police DNA lab in the country. Even so, the scale of the required genetic testing is so big that New York officials looked for help. They reached out to the New York State Police forensic DNA laboratory in Albany, and two of the world's largest genetic-sequencing companies: Myriad Genetic Laboratories Inc. in Salt Lake City and Celera Genomics Inc. in Rockville, Md. A third company, Applied Biosystems Inc. in the San Francisco Bay Area, is providing equipment and other support to the identification effort.

Spread across four laboratories, the testing process is delicate and easily compromised and must adhere to standards of evidence that can hold up in federal court, should any of the results be challenged or become part of a criminal proceeding.

Each DNA sample will take about 10 hours to extract, isolate, amplify and analyze.

So far, officials have collected 6,063 DNA samples that might help identify about 2,100 victims, New York City Police Commissioner Bernard Kerik said Monday. Dr. Robert Shaler, director of forensic biology for the medical examiner's office, is preparing to perform DNA testing on as many as 20,000 tissue samples at his laboratories. Forensic experts say that may only be the beginning.

"There are going to be a lot of unassigned parts," said Murray Marks, a forensic anthropologist at the University of Tennessee at Knoxville. "That will take a lot of DNA work; months and months of work."

Results from the four laboratories will be cross-checked continually to avoid any wrong identifications.

However long it takes, the effort is worthwhile, said Harrell Gill-King, a noted forensic anthropologist who runs the laboratory for human identification at the University of North Texas.

"We don't do forensic science for the dead," Gill-King said. "We do it for the living."

Information from Knight-Ridder Newspapers is included in this report.