(Reuters) - A blast that killed 15 people three years ago triggered by a fire at a Texas fertilizer plant was a criminal act, U.S. agents said on Wednesday after conducting extensive tests that eliminated accidental and natural causes.

“The only hypothesis that could not be eliminated ... and was confirmed by extensive testing ... is incendiary,” Bureau of Alcohol, Tobacco, Firearms and Explosives investigator Rob Elder, special agent in charge for the Houston field office. He spoke at a news conference on the blast that ripped through the city of West, about 75 miles (120 km) south of Dallas, in 2013.

Twelve of the 15 people who died were firefighters. Scores of others were injured and more than 500 homes were damaged in the blast, which was so powerful it registered as a magnitude 2.1 earthquake. Damage estimates have been pegged at $100 million.

Elder said a reward of up to $50,000 was being offered for information leading to the person or persons responsible for starting the blaze, adding that no arrests have been made.

“We have never stopped investigating this fire,” he said. “It is our highest priority to see that the victims of this tragedy are provided an accurate explanation of what happened that day.”

If a suspect is found, that person could face state capital murder charges in Texas, which can bring the death penalty. The investigation is a joint state and federal effort.

The Texas attorney general’s office said it can assist in finding and prosecuting suspects.

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“Our office stands ready and willing to assist local prosecutors in pursuing the potential criminal acts behind this catastrophe that claimed the lives of 15 Texans,” David Maxwell, the office’s director of law enforcement, said in a statement.

The explosion damaged an area measuring roughly the size of 37 city blocks, Elder said, and left a crater 93 feet (28 meters) wide by 12 feet (3.7 meters) deep.

More than 400 interviews have been conducted and more than $2 million spent on the ongoing probe, Elder said. The costs included rebuilding to exact specifications parts of the plant to determine what happened.

The fire was reported on the evening of April 17, 2013, and a large explosion ripped through the plant some 22 minutes later, Elder said.

The source of the explosion was ammonium nitrate stored in a wooden container at the plant, investigators have said.

The ammonium nitrate detonated with the force of approximately 15,000 pounds to 20,000 pounds (6,800 kg to 9,100 kg) of TNT, according to federal officials.

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The 12 firefighters who died were unprepared for the ferocity of the fire, which was “significantly beyond the extinguishment stage” and should have focused on evacuating the area rather than putting out the blaze, a 2014 report from the Texas State Fire Marshal’s Office said.

The blast obliterated an entire neighborhood - including the high school and a nursing home - on the north side of the town, where the plant had been operating for more than 50 years.

The U.S. Chemical Safety Board said in a report this year the explosion likely happened because the owner of the facility kept combustible material near a 30-ton pile of ammonium nitrate, a fertilizer also used to make explosives.

The federal investigators also faulted community planning that allowed the town to grow up around the facility, exacerbating the damage.

A Reuters investigation conducted after the explosion found that hundreds of schools, 20 hospitals and 13 churches, as well as hundreds of thousands of households, were located near ammonium nitrate storage sites across the United States.

At least seven lawsuits had been filed against Adair Grain Inc, which owned the facility. Plaintiffs claimed negligence by the plant employees and sought millions of dollars in claims.

A partial settlement that included the families of the three killed in the explosion who were not firefighters was reached in October but the details were not released.