OKLAHOMA CITY, OK — Oklahoma City was blasted twice by the same bomb April 19.

When the bomb went off, it sent out a shock wave that smashed into buildings for blocks around. A powerful vacuum formed behind it, sucking back all the air forced out by the blast. Buildings were smashed again by this second wave of destruction.

And it all happened in less than a second.

The bomb formed a gas that exploded at more than 7,000 mph. Glass splinters shot forth many times faster than a rifle bullet, then literally turned around and were embedded in walls or in the people who stood in the way.

Cement floors flew upward, then fell to the ground.

Phones were ripped from walls, walls torn from their supports. Small objects on desks became missiles.

As evidence emerged from the rubble of the Alfred P. Murrah Federal Building, investigators and researchers reconstructed what happened.

The devastation was caused by the same phenomena that occur with every bomb, says Michael Morris, who researches major vehicle bombings.

In an instant, the gas dissipates, forming a vacuum that reverses the force.

The explosion generates superhot gas that bursts outward. In that imperceptible instant, the gas in that first shock wave dissipates, forming a vacuum that reverses the force.

"I have seen high-speed film where you see a window pane going in one direction in a blast, then the same window pane reverses course and comes flying back and kills people," Morris says.

The Oklahoma City bomb was a mixture of ammonium nitrate and fuel oil, or ANFO.

Such a bomb forms a gas that explodes at more than 7,000 mph, Morris says.

Anyone 10 ft away was hit with 500 lbs per square inch of force. Yet that's slow compared with more sophisticated compounds. High-order military explosives, such as C4 and Semtex, blast gas almost three times as fast.

Still, anyone 10 feet from the two-ton Oklahoma City bomb would have been hit with nearly 500 pounds per square inch of force — equal to hitting a one-by-one-foot object with 36 tons.

At 100 feet away, that object would be hit with 3.6 tons.

As the gas hit the front of the federal building, part of the blast was deflected upward by the resistance. The front part of the floors above the blast broke and fell to the ground.

Within about a mile, 10 buildings were at least partly collapsed; 25 more had structural damage. And 312 had minor damage like broken glass.

Oklahoma City was fortunate in one respect, Morris says. A parking lot was directly across the street from the bomb, giving the gas about 250 feet to lose energy before hitting the Journal Record building. Several people inside were seriously hurt but none killed.

"You cannot direct the explosion (of a truck bomb) toward a particular target," Morris says. If there had been a building directly across the street from the federal building, the truck bomb would have wrecked it and caused many more deaths, he says. A block north, "walls were littered with shrapnel ... The top of the #4 key was shaved off my computer."

People were killed in three buildings — Murrah and the two nearest structures. Two deaths were in Oklahoma Water Resources Board offices, west of the parking lot. About 70 people were inside, but the corner on the first floor nearest the bomb was vacant.

"The majority of our people were injured. Several were in intensive care," says Duane Smith, assistant director. "All of the windows are blown in, the walls are down, the ceilings are down."

Even more than 500 feet away, on the eighth floor of a Southwestern Bell building, "the blast went through the windows and tore the walls off the studs," says spokesman Thom Hunter.

In Hunter's office, a block north in a second Southwestern Bell building, "the walls were just littered with shrapnel ... The top of the number 4 key was shaved off my computer. The chair I would have been sitting in was full of glass."

In addition to the gas blast itself, the shock wave from the bomb compressed and pushed away the air in its path, increasing pressure inside buildings and bowing walls outward.

At many buildings, "On the side ... adjacent to the bomb, all the glass blew inside," says Paul Brum, Oklahoma City public works director. "On the sides away from the bomb, most of the glass blew out."

Glass and debris accounted for many of the injuries to the 467 people who sought treatment. There is evidence in victims' wounds of how the air changed directions.

"I've seen people with glass injuries on the front, back and sides of their bodies," says Tom Coniglione, medical director of St. Anthony Hospital.

Many victims were hurt by the shock wave. They had fractures, internal injuries, ruptured ear drums and a combination of devastating multiple injuries.

Although it was too fast for people to hear, the bomb actually banged twice, Morris says. Once when it exploded, and again when the air rushing to fill the vacuum smashed together in the middle, the way thunder follows lightning splitting the atmosphere.