Two towers of 110 storeys each, collapsed into dust. Two commercial jetliners filled with passengers, crew, suicide hijackers and fuel.

A fire that probably got as hot as 2,000 C.

And the remains of as many as 5,000 bodies, some no more than ash.

How to start identifying them? The task is gargantuan, says Lawrence Kobilinsky, a professor of forensic science at the John Jay College of Criminal Justice at The City University of New York.

"You're going to have all kinds of body parts mixed with jet fuel and rubble," he said. "It seems to me from the sheer magnitude, this is the worst crime scene ever."

Prof. Kobilinsky said one of the greatest problems is the terrible heat generated when all that airplane fuel caught fire, turning the passenger planes into giant bombs.

"You're talking about the kinds of temperatures that melt steel girders," he said. "It is incredible, incredible thermal damage."

That intensity of heat creates a crematorium, burning human flesh and bone to cinders. And that makes it impossible to identify people even by using DNA.

"The reality is, if everything is pulverized and cremated, you can't test ashes," he said.

John Butt agreed. He runs the private forensic-medicine consulting firm Pathfinder Forum in Vancouver and was the chief medical examiner of Nova Scotia when Swissair Flight 111 crashed into the ocean in 1998. He was in charge of the mortuary and identification. All 229 people on board perished and all were eventually identified by DNA and other means.

But he said when it comes to charred remains, DNA doesn't work because the intense heat destroys the protein matrix in bone, leaving only minerals. That makes one person's bone indistinguishable from another's.

He said he expects that the bodies of the hijackers and others who were in the airplanes will have experienced the worst of the incendiary attack.

James Young, Ontario's chief coroner and another veteran forensic investigator from the Swissair crash, said he expects that some of the victims' bodies in New York have been entirely destroyed. That can happen either through heat or through a combination of heat and explosion.

Dr. Young was on his way to New York yesterday to help with identification of the victims.

He said he expects the process to take many months and that it is unlikely that each missing person will be accounted for and identified.

He said the task rests on meticulous record-keeping.

For example, when remains are recovered, they and anything significant near them must be put into a body bag. The contents are taken to the morgue in refrigerated trucks and then minutely catalogued.

The forensic team will then examine the body, taking note of any jewellery, tattoos, scars or birthmarks that might prove useful.

If necessary, X-rays and fingerprints are taken, as well as any information that could be matched later to dental records. A sample of DNA is also taken at that time, and so is a blood sample -- if possible -- for future toxicological tests.

One of the crucial issues will be to decide when to use DNA testing.

During the investigation of the Swissair crash, for example, the forensic team decided from the start not to do DNA testing on any body part that was reduced to just bone, Dr. Butt said.

Dr. Young said that testing every tiny shard for DNA can dramatically lengthen the time it will take to make the identifications.

"If you do too much DNA sampling, you'll swamp all the labs," Dr. Young said. "The more DNA you test, the longer it takes."