A fertilizer plant exploded in the small Central Texas farming community of West, about 20 miles north of Waco, leaving as many as 15 people dead and at least 160 injured.
A look at the facts:
Three to five volunteer firefighters and a constable responded at 7:29 p.m. Wednesday to a blaze at the West Fertilizer Co. Firefighters recognized the seriousness of the situation and began evacuating people from the immediate area. About 20 minutes later, an explosion with the force of a small earthquake shook the ground and could be heard dozens of miles away. Flames and a huge mushroom cloud filled the sky. A four-to-five block area around the plant was leveled, destroying 50 to 75 homes, a middle school and a nursing home where 133 residents, some in wheelchairs, were trapped in rubble and evacuated. The main fire was under control by 11 p.m.
Police said the death toll remained uncertain. Waco police Sgt. William Swanton estimated five to 15 deaths, among them the three to five firefighters who initially responded to the plant fire. More than 160 people suffered injuries such as broken bones, bruises, lacerations, respiratory distress, and some head injuries and minor burns.
Dozens of emergency vehicles amassed at the scene in the hours after the blast. Firefighters used flashlights to search the area for victims and survivors. A football field was used as a triage center. American Red Cross crews from across Texas headed to the scene to help evacuated residents. About 20 members of the Texas National Guard were sent to the scene and Gov. Rick Perry activated Texas Task Force 1, an 80-member urban search and rescue team equipped with heavy rescue equipment.
Authorities say there is no indication the blast was anything but an industrial accident. But they still have not been able to get to the heart of the blast site.
WHY WAS THE BLAST SO BIG?
Investigators still are looking into the exact cause of the blast. But ammonium nitrate is used commonly as fertilizer because of its high nitrogen content that fuels plant growth, said Ronald Smaldone, an assistant professor of chemistry at the University of Texas at Dallas. It's also used as a commercial explosive for mining and excavating because it's much more stable than dynamite.
Compounds with high nitrogen content become explosive under the right conditions because they form nitrogen gas as a byproduct.
If stressed, its chemical elements want to decompose into water and laughing gas, but the way they break apart is with a runaway explosive chemical reaction. "The hotter it is the faster the reaction will happen," said Neil Donahue, professor of chemistry at Carnegie Mellon University.
Ammonium nitrate is best known as the explosive used in the Oklahoma City bombing in 1995.
More than a dozen other explosions involving the chemical have occurred over the past century. The deadliest was exactly 66 years ago this week, on April 16, 1947, when a series of explosions that began with a blast on a French freighter filled with more than 2,000 tons of ammonium nitrate fertilizer rocked the huge waterfront petrochemical complex at Texas City, just southeast of Houston. At least 576 people were killed and 5,000 injured.
Others in the United States included 14 killed in Roseburg, Ore., in 1959; six firemen in Kansas City, Mo., in 1988; four people in Port Neal, Iowa, in 1994. In Belgium in 1942, 189 people were killed. In 2001, an explosion at a hangar containing 300 tons of ammonium nitrate at a chemical and fertilizer plant killed 31 people and injured more than 2,000 in Toulouse, France. Another in France killed 29 in 1947, 162 in North Korea in 2004, 37 in Mexico in 2007, and 18 in Romania in 2004.
The U.S. Chemical Safety Board was deploying a large investigation team. The board has not investigated a fertilizer plant explosion before, according to safety board managing director Daniel Horowitz. The federal Bureau of Alcohol, Tobacco, Firearms and Explosives was sending a national response team composed of investigators, certified explosives specialists, chemists, canines and forensic specialists.
Associated Press writers Malcolm Ritter in New York and Seth Borenstein in Washington contributed to this report.