TERROR IN OKLAHOMA CITY : Painstaking Work Ahead as Investigators Piece Together Crime Scene : Probe: The nation’s top experts have already been dispatched to Oklahoma City. Evidence so far suggests sophisticated group carried out attack.

WASHINGTON — Convinced that a massive car bomb was responsible for destroying the federal building in downtown Oklahoma City, federal investigators scrambled Wednesday to begin the painstaking process of reconstructing exactly what happened to determine who is responsible.

As President Clinton quickly responded Wednesday afternoon by rushing teams of investigators and bomb experts from the FBI, the Bureau of Alcohol, Tobacco and Firearms and other agencies, Atty. Gen. Janet Reno grimly vowed that the government would seek the death penalty for what Clinton called “an act of cowardice.”

Reno and other senior law enforcement officials refused to publicly speculate on a motive or on possible suspects for the crime. But privately officials clearly were struck by the similarities between Wednesday’s devastating bombing and the brutal February, 1993, attack on the World Trade Center in New York. That attack, carried out by Islamic terrorists, was also caused by an enormous car bomb--and the parallels prompted federal officials to treat Wednesday’s bombing as a terrorist attack.

Complicating the debate over a possible motive or foreign terrorist involvement, however, was the fact that Wednesday was the second anniversary of the federal raid on the Branch Davidian compound in Waco, Tex.

But it was clear from the scale of the attack--perhaps the worst terrorist act in American history--that it must have been carried out by a relatively sophisticated group. Officials at the ATF said they had determined that the Oklahoma City explosion was caused by a 1,000-pound bomb. ATF Director John Magaw said that “any time you have this kind of damage, this kind of explosion, you have to look there (at terrorists) first.”

“This was the most devastating terrorist attack we’ve ever had,” added Oliver B. (Buck) Revell, a former FBI anti-terrorism official.

The FBI called on the Central Intelligence Agency on Wednesday to cull its international sources to search for possible leads among foreign terrorist groups, CIA officials confirmed. The agency’s counterterrorism center issued a directive to all of the CIA’s stations around the world for help in the case.

But ATF and other officials cautioned that it still is too soon for them to determine exactly what type of explosive was used and whether it matched the type used in the World Trade Center case. Two of the ATF’s national response teams--which handled the World Trade Center case--arrived in Oklahoma City on Wednesday night. They were not scheduled to begin a comprehensive investigation until this morning, an ATF spokesman said.

“Unless we get a lucky break, this is going to be a long and painstaking process” of investigation, an ATF spokesman said. He refused to say whether the ATF has yet been able to identify the type of vehicle that carried the bomb.

But as the World Trade Center case proved, reconstructing the vehicle could prove to be critical to the federal investigation. An ATF official noted Wednesday that investigators obtained a major break in the World Trade Center case when experts determined the vehicle identification number from the van that carried the bomb--and that ultimately led them to those who carried out the attack.

The ATF teams will be one part of a legion of federal law enforcement officials investigating the attack. The President ordered White House Chief of Staff Leon E. Panetta to coordinate an interagency group to oversee the federal response. Clinton designated the FBI as lead agency. Four FBI special agents in charge from around the country were setting up a 24-hour command post to supervise the work of four evidence-response and explosive-ordnance teams. Bomb technicians and five of the bureau’s most experienced investigators in bombings also are part of the team.

Meanwhile, 13 members of the FBI’s rapid start team will be logging evidence as it is collected and 50 more agents are scheduled to arrive today. The Secret Service sent experts in explosives, while the U.S. Army’s 61st Ordnance Detachment from Ft. Sill, Okla., was being deployed as well.

Bomb technicians from the ATF and other agencies are likely to play the most critical role in the initial stages of the investigation, as officials seek to reconstruct the bomb, the vehicle and related evidence. Indeed, despite the enormous energy of the blast, pieces of the bomb and vehicle will remain at the site. The trick will be for investigators to find them and reconstruct them into usable evidence.

But until rescue operations are completed and the damaged structure is stabilized, bomb investigators are likely to restrict their initial search for clues to the blast perimeter.

But there is much to look for there--from telltale parts of the vehicle that carried the explosive to residue revealing the ingredients of the bomb.

One of the most important clues could be the detonation device or telltale signs of ammonium nitrate and fuel oil, ingredients used in previous terrorist car bombings. Not only would the detonator or the ingredients help determine the sophistication of the bomb, but they could help investigators pinpoint likely suspects. Some terror groups tend to use specific types of timers and detonators.

The first thing that leaders of the bomb teams will do is establish a formal investigative plan, breaking down the massive crime scene into a series of small scenes--each with its own team of investigators.

Using tools that are likely to include brooms, shovels and sifters capable of retrieving an earring post, they will begin examining rubble, dirt and dust debris. Some of the most important clues could be no bigger than a pencil eraser.

With sterile gauze and acetate, they will swab walls and solid surfaces to recover explosives residue.

Malcolm Brady, head of the ATF bomb team that solved the World Trade Center case, said in an interview at that time: “What a lot of people don’t understand is that an explosion doesn’t destroy the evidence--it just makes it smaller.”

Meanwhile, a spokesman for the Oklahoma City Police Department said Wednesday night that the FBI had given police officials a witness description of three men in a brown Chevrolet leaving the scene shortly before the explosion. FBI officials later discounted that report.

Times staff writers Ronald J. Ostrow and William C. Rempel contributed to this story.