WEST, Texas (AP) — Rescuers searched the smoking remnants of a Texas farm town Thursday for survivors of a thunderous fertilizer plant explosion, gingerly checking smashed houses and apartments for anyone still trapped in debris or bodies of the dead. The accident killed as many as 15 people and injured more than 160 others.

Daylight revealed a breathtaking band of destruction extending for a four- or five-block radius around the West Fertilizer Co. in the small community of West, about 20 miles north of Waco. The blast shook the ground with the strength of a small earthquake and leveled homes, apartments, a school and a nursing home. Its dull boom could be heard dozens of miles away.

Waco police Sgt. William Patrick Swanton described ongoing search-and-rescue efforts as "tedious and time-consuming," noting crews had to shore up much of the wreckage before going in.

Searchers "have not gotten to the point of no return where they don't think that there's anybody still alive," Swanton said. He did not know how many people had been rescued.

There was no indication the blast, which sent up a mushroom-shaped plume of smoke and left behind a crater, was anything other than an industrial accident, he said.

The Wednesday night explosion rained burning embers and debris down on terrified residents. Morning exposed a landscape wrapped in acrid smoke and strewn with the shattered remains of buildings, furniture and personal belongings.

Dogs with collars but no owners trotted nervously through deserted streets in cordoned-off neighborhoods around the decimated plant. The entire second floor of a nearby apartment complex was destroyed, leaving bricks and mattresses among the rubble. One rescue crew going from apartment to apartment gave special attention to a room where only a child's red and blue bunk bed remained.

While the community tended to its deep wounds, investigators awaited clearance to enter the blast zone for clues to what set off the plant's huge stockpile of volatile chemicals.

"It's still too hot to get in there," said Franceska Perot, a spokeswoman for the Bureau of Alcohol, Tobacco and Firearms.

The precise death toll was uncertain. Three to five volunteer firefighters were believed to be among the dead, which authorities said could number as many as 15. But that was merely an estimate.

Swanton said he would "never second-guess" firefighters' decision to enter the plant because "we risk our lives every day." The many injuries included broken bones, cuts and bruises, respiratory problems and minor burns. Five people were reported in intensive care.

In the hours after the blast, residents wandered the dark, windy streets searching for shelter. Among them was Julie Zahirniako, who said she and her son, Anthony, had been at a school playground near the plant when the explosion hit.

The explosion threw her son four feet in the air, breaking his ribs. She said she saw people running from the nursing home, and the roof of the school rose into the sky.

"The fire was so high," she said. "It was just as loud as it could be. The ground and everything was shaking."

First-responders evacuated 133 patients from the nursing home, some in wheelchairs. Many were dazed and panicked and did not know what happened.

William Burch and his wife, a retired Air Force nurse, entered the damaged nursing home before first-responders arrived. They searched separate wings and found residents in wheelchairs trapped in their rooms. The halls were dark, and the ceilings had collapsed. Water filled the hallways. Electrical wires hung eerily from the ceilings.

"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, said Neil Donahue, professor of chemistry at Carnegie Mellon University.

"The hotter it is, the faster the reaction will happen," Donahue 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.

There were no immediate details on the number of people who work at the plant, which state environmental regulators last inspected in 2006, said Zak Covar, executive director of the Texas Commission on Environmental Quality. A complaint about a strong ammonia smell triggered that investigation, which the company resolved by obtaining a new permit.

Covar said no other complaints are on file since then, so there haven't been additional inspections.

"Given the size of the facility and the authorized emissions from the plant, generally those are inspected based on complaints," Covar said.

The Chemical Safety Board has not investigated a fertilizer plant explosion before, but Managing Director Daniel Horowitz said "fertilizers have been involved in some of the most severe accidents of the past century."

He noted the 2001 explosion at a chemical and fertilizer plant that killed 31 people and injured more than 2,000 in Toulouse, France. The blast in a hangar containing 300 tons of ammonium nitrate came 10 days after the Sept. 11, 2001, terrorist attacks, raising fears at the time that the two could be linked. A 2006 report blamed the blast on negligence.

Horowitz also mentioned a disaster in Texas City in 1947, when a fire in a cargo ship holding more than 2,000 tons of ammonium nitrate caught fire and exploded, killing more than 500 people.

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