Investigators: Texas plant fire cause unknown

WEST, Texas -- Investigators have completed their scene investigation but not ruled out criminal activity as the cause of a massive explosion at a fertilizer plant that killed 15 people and flattened part of a tiny Texas town, officials said Thursday.

The April 17 blast at West Fertilizer injured 200 and leveled part of the tiny town of West. Officials have spent one month combing through debris and speaking to hundreds of witnesses.

"At this time, the state fire marshal's office and ATF are ruling the cause of this fire is undetermined," State Fire Marshal Chris Connealy said at a news conference. A criminal investigation continues.

Possible causes of the fire that triggered two explosions have been narrowed to a 120-volt electrical system at the plant, a golf cart or an intentionally set fire, officials said.

Kelly Kistner, assistant state fire marshal, said investigators estimated that between 28 and 34 tons of ammonium nitrate on the site exploded. Kistner said it was the equivalent of 15,000 to 20,000 pounds of dynamite. Investigators had ruled out other possible causes, including smoking or a weather-related fire.