Deadly Mix of Fertilizer, Oil Detailed by FBI

Criminal investigators sifting through the rock and iron rubble in downtown Oklahoma City determined Thursday that several thousand pounds of a “low-order” but highly deadly mixture of fertilizer and fuel oil was used in the explosion that blew apart the federal office building here.

Disclosing that the explosive materials were ferried in the back of a large rented truck, authorities said they have now recovered the truck’s axle and hundreds of tiny shrapnel pieces strewn across several city blocks around the Alfred P. Murrah Federal Building.

In addition, bomb experts and evidence technicians are operating on a theory that an elaborate “one-on-one” explosive device was wired inside the truck and used to set off one detonation, which then triggered a second, massive explosion.

But perhaps more important, sources said, the double-trigger mechanism might have given the bombers--possibly two unidentified white men who rented the truck earlier this month--enough time to get out of the vehicle and rush safely away before the blast tore through the landmark building.

They said preliminary evidence also shows that the truck, believed to be a Ryder rental from Junction City, Kan., was driven up a circular driveway at the main entrance and then abandoned in a no-parking, no-standing zone in front of the nine-story structure.

But it appears that the building’s video cameras would not have captured the vehicle’s arrival because the cameras were turned toward the building and not out at the driveway. It may never be known for sure how long the vehicle was there before it may have been recognized or the bomb went off.

The explosion rocked the normally quiet downtown section shortly after 9 a.m. Wednesday, knocking down parts of all nine stories of the office building, creating a 30-foot-wide crater and scattering debris throughout the area. Office windows were shattered and cars damaged throughout the downtown business section, and people reported hearing the blast from as far away as 15 miles.

Dozens of bodies of adults and children were believed still lying inside the devastation Thursday. The death toll is now expected to climb to more than 200 by the time fire and rescue officials have completed their grim search by this weekend. The number of injured is reported at more than 400 already.

Federal investigators here were moving on several fronts on their first full day after the explosion. They were chasing a wealth of leads in what, if eventually proven, would be the worst single terrorist attack on U.S. soil. But they also cautioned that no positive links have been found to foreign terrorist groups.

They also suggested Thursday that the assailants could turn out to be U.S. citizens seeking revenge against local federal law enforcement officials who worked inside the office structure.

Along with announcing the search for the two white men, they also confirmed reports that a third man identified as a “possible witness” had been detained with a U.S. passport at London’s Heathrow Airport and was returned to the United States.

Weldon Kennedy, an FBI special agent-in-charge sent here from Washington to command the investigation, said that “the explosive used in this consisted of fertilizer and fuel oil. We know that from the crime scene itself.

“Forensic experts who have examined the scene thus far are able to say that it is a low-order explosive. Not a high explosive. They can tell that from the damage to vehicles and other surrounding buildings nearby.” A low-order explosive is one that usually causes the most damage at its ignition site rather than exploding throughout a large area, Kennedy said.

Authorities reached their conclusion partly because of the huge crater left where the truck had exploded.

But even with these new revelations, Kennedy said, the probe is far from complete.

“Can I tell the American public this morning that we are any closer to finding out who did this than we were yesterday?” he asked. “The answer to that question is, no, I cannot.”

But Kennedy did say that so far federal investigators believe only one vehicle was used to carry one explosive mechanism, discounting some reports that another vehicle may have ferried a second bomb.

“To my information we only have one device,” he said. “No others have been found.”

Oklahoma Gov. Frank Keating, a former FBI agent, U.S. district attorney in northern Oklahoma and head of the Bureau of Alcohol, Tobacco and Firearms in Washington, said a large truck--and not an open-bed pickup--would have had to be used to carry the several thousand pounds of fertilizer and fuel oil.

He said the axle and other debris have been located from what is believed to be the attack vehicle.

“The axle that I saw from that car was two blocks to the east,” he said. “And there are little pieces not any bigger than your fist that fell from that car. Broken, charred metal. And it was an enormously powerful blast.”

Could the bombers have been inside the vehicle or been killed before they got safely away? “That is certainly something that will only be determined with difficulty,” he said. “There was not much left.”

He said the no-parking, no-standing zone fronting the federal building gave the bombers a special vantage point. “If you wanted to do damage to the building, that was the best place for a car,” he said.

But despite the difficult task of reassembling the facts, the governor remained optimistic that arrests will be made.

“This case will be solved,” he said. “Every bomb has a signature. And these types of crimes are solvable.”

Washington has sent an impressive team of investigators here to work with local authorities. They include 13 members of the FBI’s rapid start team and a second contingent of 50 agents who will assist with the probe.

The ATF dispatched two response teams and a mobile command center used by explosives experts and laboratory technicians. Another two dozen ATF agents were also arriving.

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Aiming a Bomb

The direction and shape of an explosive charge are critical factors in how much damage a bomb can do.

BLAST DIRECTION

The direction of a bomb blast, like the one in Oklahoma City, can be controlled by using a shield. A shield can be anything from a line of sandbags to reinforced metal sidings. The Oklahoma device was far more effective than the February, 1993, World Trade Center bomb.

World Trade Center: The New York City bomb was not packed to go off in any single direction, but in all directions equally. As a result, its main impact was down; it left a crater six stories deep. The reinforcement above the garage managed to stifle the impact. If it had been packed properly, it could have been far more lethal in damaging the above ground structure.

Oklahoma City: The Oklahoma City bomb appears to have been packed in such a way that the explosiion would target one side, rather than all sides, thus having a greater impact on the front of the Alfred P. Murrah building.

Types of Bombs:

Dynamite (Nitroglycerin and sawdust): Creates strong carbon odor and leaves nitrate residue and pieces of paper wrappings. Tears metal into fragments several inches in size.

ANFO (Diesel oil and fertilizer): Creates fuel oil odor and heavy black smoke. Also leaves nitrates and large metal fragments.

C-4 (Cyclotrimethylene and plastic bonding): Leaves nitrates, unless detonated by another explosive such as dynamite. Leaves small undetonated particles of plastique. Tears metal into tiny pieces.

SEMTEX (Cyclotrimethylene, pentaerythritol tetronitrate and plastic): Leaves distinctive nitrate residue, nearly no odor. Leaves particles of plastique and tears metal into tiny pieces.

Source: Los Angeles Times; Newsday; Clark Staten, Emergency Response and Research Institute; Researched by DAVID F. MONTESINO and ROBIN WRIGHT / Los Angeles Times