WEST -- Investigators have revealed that the fire that caused an April 2013 explosion at a fertilizer plant in West, Texas was intentional.

During a press conference Wednesday, officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives called the fire "a criminal act."

“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” at the ATF laboratory in Maryland, ATF special agent in charge Rob Elder said Wednesday.

Press conference in West on May 11, 2016

The fire was reported at the facility around 7:30 p.m. on April 17, 2013. A large explosion occurred roughly 20 minutes later.

Fifteen people were killed in the explosion, including 12 first responders. Another 160 were injured. At least 500 homes were destroyed in a 37-square-block area.

Evidence was collected from as far away as two-and-a-half miles from the fertilizer plant, Elder said.

Elder addressed the families of those killed in the explosion Wednesday.

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

Over $2 million has been spent on the investigation of the fire. It's one of the ATF's largest fire investigations in history in terms of money spent as well as the scope and man hours spent, according to the bureau.

No arrests have been made in the investigation, but the ATF has established "many leads" through the hundreds of interviews, Elder said.

"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 mind set," said Kelly Pustejovsky, the widow of one of the firefighters killed.

The ATF is offering a reward of up to $50,000 for information that leads to an arrest in the case. An additional $2,000 reward is being offered by Crime Stoppers.

KVUE contributed to this report.