WEST, Texas - Authorities say the fire that caused the deadly explosion at a Texas fertilizer plant in 2013 was a criminal act.
Federal and state investigators said Wednesday that they're investigating who was responsible for causing the fire at the West Fertilizer Co. facility on April 17, 2013, in the town of West.
The fire caused ammonium nitrate to ignite, triggering a massive explosion that killed 15 people, injured hundreds and left part of the small town in ruins.
The U.S. Chemical Safety Board and the state fire marshal's office previously issued reports faulting the storage of the fertilizer, emergency response and other factors contributing to a blast, which registered as an earthquake of magnitude 2.1.
But authorities hadn't previously indicated what specifically caused the fire.
Mystery is over
The three-year mystery of what caused a Texas fertilizer plant explosion that killed 15 people has come to an end.
BREAKING: ATF announces $50K reward in West Fertilizer fire. — ATF HQ (@ATFHQ) May 11, 2016
Officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives and the Texas State Fire Marshal's Office revealed the cause of fire to be arson.
Families of the those killed have struggled to find answers to what happened on April 17, 2013.
That night, a fire broke out at the West Fertilizer Co. in the town of West, about 70 miles south of Dallas. Twenty minutes later, the plant exploded with such force it caused a magnitude-2.1 earthquake.
"It was like a nuclear bomb went off," West Mayor Tommy Muska said.
A deafening boom echoed for miles. The blast stripped a 50-unit apartment complex of its walls and windows.
It was "massive -- just like Iraq, just like the Murrah (Federal) Building in Oklahoma City," said D.L. Wilson of the Texas Department of Public Safety.
The blast also left a crater almost 100 feet wide and 10 feet deep.
Twelve emergency responders were among the 15 people killed. The explosion wounded another 200 people in the town of 2,800.
Possible, impossible causes
About a month after the blast, the state fire marshal's office said four potential causes -- weather, natural causes, anhydrous ammonium, and ammonium nitrate in a rail car -- had been eliminated.
But State Fire Marshal Chris Connealy said investigators were not able to rule out three possible causes, including a spark from a golf cart, an electrical short or an intentionally set fire.
The West Fertilizer Co., which operated the facility, had warned state and local officials but not federal agencies that it had 270 tons of highly volatile ammonium nitrate on site, according to regulatory records.
The company had been cited by federal regulators twice since 2006.
A U.S. Chemical Safety Board investigation concluded the explosion was preventable, board chairman Rafael Moure-Eraso said on the one-year anniversary of the blast.
The investigation blamed the company that owned the fertilizer plant, government regulators and other authorities for the catastrophe.
"It resulted from the failure of a company to take the necessary steps to avert a preventable fire and explosion and from the inability of federal, state and local regulatory agencies to identify a serious hazard and correct it," Moure-Eraso said.
WATCH: Engineering Disasters: West Fertilizer Plant Explosion | History Channel
Thousands in fines
In 2012, the Transportation Department's Pipeline and Hazardous Materials Safety Administration fined West Fertilizer $5,250 for storing anhydrous ammonia in tanks that lacked the proper warning labels.
The agency originally recommended a $10,000 penalty, but it was reduced after the company took corrective action.
In 2006, the EPA fined the company $2,300 and told its owners to correct problems that included a failure to file a risk management program plan on time.
The Texas Commission on Environmental Quality also investigated a complaint about the lingering smell of ammonia around the plant the same year.
Victims as heroes
The blast was so catastrophic, it leveled houses for blocks around.
"It demolished both the houses there -- mine and my mom's -- and it killed my dog," said Cheryl Marak, who lived nearby, hours after the explosion.
But her husband Marty, a volunteer firefighter, had no time for panic or grief that night. He sped right toward the danger, even as the threat of a second explosion loomed.
A flood of other volunteers also scrambled to the scene, including firefighters and emergency medical personnel from hundreds of miles away, the Texas Department of Public Safety said.
Like the rest of the firefighters in West, Marty Marak wasn't getting paid to help save his community.
"That's just the way that we Texans are wired," said Rep. Bill Flores, whose district represents West. "Even though we face our own personal tragedies from time to time, we still know that we have to go help others -- and then assess our own tragedies later on."