Doug Stanglin, and Rick Jervis

USA TODAY

Federal investigators on Wednesday blamed arson for a deadly explosion at a fertilizer plant in Texas in 2013 that killed 15 people, calling it a "criminal act."

The dead included 12 first responders who rushed to the scene of the blast in the small community of West, Texas, about 75 miles south of Dallas.

The explosion, which occurred about 20 minutes after a fire was reported at the West Fertilizer Company storage and distribution facility plant, also injured 160 people and destroyed 500 homes over a 37-square-block area. Authorities collected evidence from as far away as two and a half miles, officials said.

The U.S. Chemical Safety and Hazard Investigation Board called it "one of the most destructive incidents ever investigated" by the organization.

There have been no arrests in the case, but authorities have "many leads" as a result of hundreds of interviews, said Rob Elder, Alcohol, Tobacco, Firearms and Explosives special agent in charge.

Investigators eliminated all reasonable accidental causes to the fire and subsequent explosion, including electrical malfunctions and a misfiring golf cart inside the complex, and narrowed the origin of the fire to the seed building inside the plant, he said.

"This was a criminal act,” Elder told reporters at a news conference in West.

Authorities spent around $2 million on the investigation, which included the rebuilding of a portion of the destroyed fertilizer plant to exact specifications.

“We have come to this conclusion through over 400 interviews, a systematic fire scene examination, considering witness observations, viewing both witness photographs and video, as well as extensive scientific testing,” Elder said.

He said investigators are on the right path and asked for the public's assistance "to help us get across the finish line.”

ATF is offering $50,000 for information leading to arrests, and the Waco-McClennan County Crimestoppers is offering an additional $2,000.

Elder also addressed the families of the victims. “Your loss is felt by ATF," he said. "It has been a driving factor of why we’ve gone to the lengths we have.”

"I pray to God that they get caught and get the death penalty. No life deserves to be lost due to someone's stupidity, criminal mindset," said Kelly Pustejovsky, the widow of one of the firefighters killed, WFAA-TV reported.