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updated 5/16/2013 8:16:22 PM ET 2013-05-17T00:16:22

Investigators concluded their scene excavation of the area damaged by last month's fertilizer plant explosion West, Texas. However, the broader investigation continues.

One month after a fertilizer plant explosion in West, Texas, killed 15 people and scarred a large swath of the surrounding area, investigators have concluded their excavation of the affected area. On Thursday, representatives from the state fire marshal’s office and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) announced that they had not yet determined the cause of the explosion, and that they had not yet ruled out sabotage.

“Only the scene investigation is complete, but the remainder of the investigation continues,” said Robert Champion, the special agent in charge at the Dallas ATF branch, at a Thursday press conference.

The explosion was triggered when a fire inside the plant detonated between 28 and 34 tons of ammonium nitrate, investigators said. Criminal activity is only one of a few possible explanations for the fire: a problem with the plant’s 120 volt electrical system or a faulty golf cart located on the premises could have also been responsible. Investigators ruled out several other possibilities over the course of their excavation, including ignition of the plant’s anhydrous ammonia and smoking on the premises.

Texas fire marshal Chris Connealy also disclosed more details about the extent of the damage caused by the explosion. The crater from the blast was 29 feet wide and 9 feet deep, he said, and damaged property within a 37-block radius.

“This community has suffered a great tragedy,” he said, “and us, with the state fire marshal’s office, the ATF, and 28-plus other agencies all work in the common goal to understand what happened.”

The ATF invested an unusual amount of time and money in the investigation: while the average call-out time for an ATF field investigation is three to seven days, this time the agency spent 30 days and nearly $1 million.

“This event here is one of our longest call-outs we’ve ever had,” said Champion, who added that it ranks up with the Oklahoma City bombing, the 1993 World Trade Center attack, and the 2001 attack on the Pentagon.

Last week, one of the first responders to the fertilizer plant explosion was arrested and charged under federal law with possession of a pipe bomb. On Thursday, investigators declined to comment on whether that arrest was related to the criminal investigation into the cause of the plant explosion.


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