The massive Texas fertilizer-plant explosion that killed 15 people and injured more than 160 others in 2013 was caused by arson, federal officials announced Wednesday.

Special Agent in Charge Rob Elder, from the Bureau of Alcohol, Tobacco, Firearms, and Explosives, said extensive scientific testing at the agency’s Maryland office ruled out all accidental causes and confirmed an incendiary—or criminal—cause. Officials were determining whether the fire at the West Fertilizer Co. and ensuing explosion was caused by an electrical problem with a golf cart, an electrical issue with the plant, or arson. “All viable accidental and natural fire scenarios were hypothesized, tested, and eliminated,” Elder said.

The investigation is still active, he indicated. “In order to protect the integrity of the investigation,” there were very few details released. Elder said more than 400 interviews have produced many leads. No arrests have been made and no suspects were announced. To date, $2 million has been spent on the investigation, officials said.

Special Agent in Charge Nicole Strong told The Daily Beast that she couldn't elaborate on whether distinct accelerant evidence led to the agency's conclusion, but said, "we do have very specific information that leads us to 'incendiary.'" In regards to a suspect, Strong said the agency believes "we're on the right track." The announcement of a $50,000 reward on Wednesday, she said, was "to help us get over that final hump."

The announcement comes more than three years after the devastating blast rocked the small town of West (population: 2,834, at last count). Twelve of those killed in the April 17 explosion were firefighters responding to the blaze when the plant erupted. The explosion destroyed more than 500 homes and a 37-square-block area, causing a crater 93 feet wide and 12 feet deep—and caused a 2.1-magnitude earthquake—in what has been called “one of the most destructive incidents ever investigated by the U.S. Chemical Safety and Hazard Investigation.”

Family members of the felled victims were invited to a private conference on Wednesday morning to hear the findings before officials addressed the media, Strong said. "Some elected not to attend," she added.

Resulting lawsuits from the catastrophe have been filed on behalf of the city’s schools, nursing home, local government, and at least 200 homeowners, the Waco Tribune-Herald reports. A third trial related to the disaster is set for July 25; all of the defendants in that case either sold or manufactured fertilizer to the company, the newspaper wrote. The first two were reportedly settled outside of court.

— Olivia Messer