An important part of the criminal investigation into last week's U.S. terror attacks is identifying the remains of thousands of people killed in New York and Washington. Law enforcement technicians are using all the tools of a modern crime laboratory to confirm who is buried in the rubble and even help trace the movements of the dead terrorists.

It is the biggest investigation in U.S. history. Federal technicians are culling hundreds of thousands of tons of debris from the collapsed World Trade Center and U.S. military headquarters to gather shreds of evidence that can help identify the dead and shed light on the terrorist acts.

Forensic scientist Lawrence Kobilinksy of the City University of New York says the task is monumental. He said, "This particular crime scene is like no other crime scene we have ever had. The magnitude is so enormous, it's hard to put words to describe exactly what a difficult, complex situation this is."

The best evidence that can come out of the mountains of rubble is bodies and accompanying papers. The more complete they are, the better like the passport of one of the hijackers that Mr. Kobilinsky says landed blocks from the World Trade Center. "Pamphlets, documents - that's the kind of evidence that investigators are looking for to find out if there is anything at all that points to the source of this attack," he said. "There is also a great deal of personal material that families would like to have."

But the airliner explosions and the collapse of the buildings surely cremated or ripped apart countless bodies. Therefore, forensic scientist Tod Burke of Radford University in Virginia says evidence collectors must focus on miniscule bits of remains. "I'm talking about hairs, fibers, blood," he said. "They may have to go to dental records. Because of the extreme fire, there may not be fingerprints."

A single tooth can identify a person. But without such evidence, investigators must turn to genetic analysis of hairs, bits of flesh, and blood. To be successful, they must have comparison DNA samples deposited before death. In the case of military casualties, their DNA is already on file.

For the rest, DNA identity expert Arthur Eisenberg of the University of North Texas Health Science Center says investigators are asking relatives to bring them personal belongings of the dead. Mr. Eisenberg said, "They have asked for things like soiled undergarments, hairbrushes, envelope flaps that a person has licked - anything [from which] you can recover a direct biological sample for comparison purposes."

When such evidence is unavailable, DNA samples from relatives are the next best thing - either blood or mucous swabbed from inside cheeks.

The airline crash sites are not the only locations being scoured. Radford University's Tod Burke says investigators are gathering evidence from places in the United States the terrorists are thought to have been. "If you can establish where they have been," he said, "you might be able to find out their associates. If the physical evidence was in someone else's apartment [or hotel room] or was shared, then someone else was involved with this or at least they had contact with that individual. You want to find out what they knew."

At the University of North Texas Health Science Center, Arthur Eisenberg says the U.S. forensic community has offered to help federal investigators. Lawrence Kobilinsky of the City University of New York says the grisly nature of the tragedy may mean the identification will never fully succeed. "There are probably several hundred thousand body parts," he said. "With that kind of damage, we will certainly not be able to identify all of the bodies. Of course, I don't think anybody really understands the anguish of the families because they will be waiting a great deal of time to find out if their loved ones have indeed been killed."

Mr. Kobilinksy expects the tedious recovery and forensic work to drag on well into next year.