The fire that triggered the massive explosion at a West, Tex., fertilizer depot last month started in a building that held the company's seed and fertilizer, according to a lead investigator into the tragic blast.
The building contained the company's store of ammonium nitrate - the volatile compound used in fertilizer and explosives - and was completely destroyed in the blast, Brian Hoback, national response team supervisor for the Bureau of Alcohol, Tobacco, Firearms and Explosives, said in an interview with USA TODAY. The bureau is leading the investigation along with the Texas State Fire Marshal's Office.
The Apr. 17 explosion at the West Fertilizer Co., killed 14 people, including 10 firefighters, injured more than 200 and destroyed dozens of homes as it decimated a five-block radius.
The fire that led to the explosion started inside the storage building but not exactly at the spot where the ammonium nitrate was stored, Hoback said.
The fire eventually reached the ammonium nitrate, which fueled the explosion, leaving behind a 93-foot-wide charred crater where the building once stood, he said. Investigators are now zeroing in on the crater, sifting through ashes and debris by hand for clues to what may have caused the initial fire.
"You have a building where fertilizer and seed are stored in there," Hoback said. "The fire was contained to that one building."
For the past two weeks, more than 60 agents from 29 agencies have run down 237 leads and interviewed more than 411 people during the query, said Kelly Kistner, assistant State Fire Marshal. Agents began searching the outermost perimeter of the blast, collecting debris from as far as 2-1/2 miles away, and meticulously worked their way toward the center, he said.
Investigators don't yet have a precise cause of the fire that triggered the explosion, but have ruled out several things, including natural events - such as lightning strikes - and stores of anhydrous ammonia, Kistner said.
Investigators also say nothing the firefighters did responding to the initial blaze caused the explosion, he said. A possible electrical fire or foul play have not been ruled out, Hoback said.
The agencies hope to wrap up the initial phase of the investigation in another week or two, he said. Sifting through the evidence has been tedious and challenging given the explosion's massive blast perimeter.
"Some of the pieces of the puzzle were completely destroyed," Hoback said. "Some were blown as far as 3,000 feet away."
Testimony from surviving firefighters who rushed to the initial blaze has been "very helpful" in piecing together what happened the evening of Apr. 17, Kistner said.
The mostly-volunteer firefighters were aware of the explosive potential of the ammonium nitrate in the building and were retreating from the property when the blast struck, West Mayor Tommy Muska said. "They knew what the situation was," he said.
Shortly after the blast, explosion experts pointed to the large quantity of ammonium nitrate stored at the site as the likely cause of the explosion. Hoback confirmed that ammonium nitrate - a white, crystalline substance used to reinvigorate fields - caused the explosion, but his teams are still trying to determine exactly how much was on the property when it blew.
The company had 270 tons of ammonium nitrate on site - or more than 100 times the amount used in the 1995 Oklahoma City bombing - as of last year, according to state records.