FILE - In this April 18, 2013 file photo, mangled debris of the West Fertilizer Co. plant are seen, a day after an explosion leveled the plant in West, Texas. Federal authorities announced Wednesday, May 11, 2016, that the fire that caused the deadly explosion in 2013 was a criminal act. The explosion killed 15 people, injured hundreds and left part of the small town in ruins. (AP Photo/Charlie Riedel, File)

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

2:45 p.m.

The mother of a volunteer firefighter who died in the massive 2013 plant explosion in West, Texas, says the state has taken steps to prevent a similar incident from happening again. But she believes something similar could occur elsewhere.

Mary Sanders's son, Kevin, was among 15 people killed when a fire at the plant caused ammonium nitrate to ignite, triggering a massive explosion. Investigators announced Wednesday that they believe the fire was intentionally set.

Sanders, who lives in the Chicago suburb of Palos Hills, said Wednesday that regulations aren't uniform from state and state. She says federal regulations are needed to prevent such catastrophic events, whether they're accidental or intentional.

After the blast, Texas adopted regulations that included ensuring ammonium nitrate be kept separate from combustible material.

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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.

News from © The Associated Press, 2016