Already the dirty, gritty business of bomb work has begun -- investigators in blue fatigues, perhaps in helmets, certainly in gloves, digging through an 8-foot-deep crater, sifting through metal pieces, glass shards and other debris to identify the explosives used in the Oklahoma City blast and track down the terrorists who planted them in a rental truck.
It's painstakingly slow work, searching for the tiniest of clues -- a granular chemical residue, a snip of electrical wire, a bit of metal from a timing piece.
As these crime archaeologists sift through the core of the crater, others will comb a four-block area for more evidence.
Along with the forensic work, investigators will rely on intelligence sources and "pure dogleg police work" to crack the case, terrorism and explosives experts said yesterday.
"What happens, you end up with a domino effect," said Alice T. McGillion, managing director of Kroll Associates and a former deputy police commissioner in New York. "You start your process looking for basic forensic evidence. It starts to take you down certain paths. And then you start to pursue those paths."
For example, an axle found two blocks from the Alfred P. Murrah Federal Building led investigators to identify a truck as the vehicle used in the attack. They traced the vehicle to a Ryder rental company.
The type of bomb can lead investigators to the manufacturer of the explosives. Its detonation device may resemble the trigger of a device used in another celebrated explosion. That, in turn, may lead to the perpetrators of the crime.
More than 87 federal investigators -- including FBI and Bureau of Alcohol, Tobacco and Firearms agents who assisted in the World Trade Center bombing -- are working the crime scene.
Once the bomb site is identified, investigators determine the perimeter to search, an area that could take them blocks from the federal center. The building is divided into quadrants with evidence teams assigned to each.
The investigators' work, however, has been complicated by the shaky condition of the nine-story building and the imperative of the day -- the search for and rescue of any victims still trapped amid the rubble, the experts said.
"The fire and rescue people are in there doing what they have to do, which is more important, but they could be picking up evidence on their shoes," said Brian C. Murphy, the director of security at John Jay College of Criminal Justice in New York and a former New York City bomb squad detective. "This is the very unglamorous part of bomb work, the really dirty part. They've got to go into that place and literally sift through every ounce of rubble."
Dan Boeh, an agent with the Bureau of Alcohol, Tobacco and Firearms in Baltimore who is assisting in the Oklahoma City investigation, said: "In a crater, you are going to have things that go down in the ground and things that go up and things that go in all different directions.
"From the workable areas, you collect everything that is in there. You sift everything that's in there and come up with items that look like the bomb or the car."
The work is tedious. It can also be grisly.
"You find body parts. You may find evidence on body parts," said Mr. Murphy.
The tools of this trade include large strainers, evidence vacuum cleaners and laboratory test equipment. The work can be as sophisticated as a chemical analysis or as simple as retrieving a twisted piece of metal that resembles a car part.
"When there is a large blast scene like this of national importance, the ATF and FBI will bring with them both a field test and analytical instrumentation that will enable them to perform many analyses at the scene that would normally be done at the lab," said Dr. Carl M. Selavka, a fellow of the American Academy of Forensic Sciences and director of criminalistics at National Medical Services in Willow Grove, Pa.