WEST, Texas — "They had Sheetrock that was on top of them. You had to remove that," Burch said. It was "completely chaotic."
Gov. Rick Perry called the explosion "a truly nightmare scenario for the community" and said he had been in touch with President Barack Obama, who promised his administration's assistance with operations on the ground.
Authorities said the plant made materials similar to those used in the 1995 Oklahoma City bombing.
The fertilizer used in that attack, ammonium nitrate, makes big explosions, be they accidental or intentional, said Neil Donahue, professor of chemistry at Carnegie Mellon University. It also was used in the first bombing attempt at the World Trade Center in 1993.
Ammonium nitrate is stable, but if its components are heated up sufficiently, they break apart in a runaway explosive chemical reaction, Donahue said.
"The hotter it is, the faster the reaction will happen," he said. "That really happens almost instantaneously, and that's what gives the tremendous force of the explosion."
About a half-hour before the blast, the town's volunteer firefighters had responded to a call at the plant, Swanton said. They immediately realized the potential for disaster because of the plant's chemical stockpile and began evacuating the surrounding area.
The blast happened 20 minutes later.
Erick Perez was playing basketball at a nearby school when the fire started. He and his friends thought nothing of it at first, but about a half-hour later, the smoke changed color. The blast threw him, his nephew and others to the ground and showered the area with hot embers, shrapnel and debris.
"The explosion was like nothing I've ever seen before," Perez said. "This town is hurt really bad."
The U.S. Chemical Safety Board was deploying a large investigation team to West. An ATF national response team that investigates all large fires and explosions was also expected, bringing fire investigators, certified explosives specialists, chemists, canines and forensic specialists. American Red Cross crews also headed to the scene to help evacuated residents.