WEST, Texas (AP) — 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.

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