The Latest on the investigation into what cause the massive 2013 explosion at a fertilizer plant that killed 15 people in the Texas town of West (all times local):

1:15 p.m.

Federal investigators say they believe a fire that caused the fatal explosion at a Texas fertilizer plant was intentionally set, but that no arrests have been made.

The Bureau of Alcohol, Tobacco, Firearms and Explosives released a statement Wednesday saying the fire had been ruled "incendiary," or intentionally set.

The agency said the conclusion was made after more than 400 interviews, a fire-scene examination, a review of witness photos, videos and observations, along with "extensive scientific testing" at an ATF fire research laboratory.

The agency says "all viable accidental and natural fire scenarios were hypothesized, tested, and eliminated."

The fire killed 15 people on April 17, 2013, most of them emergency personnel.

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12:20 p.m.

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.