The Texas fertilizer factory where an earthquake-like explosion killed up to 15 people was fined by US regulators in 2012 over its transport of hazardous materials, documents showed Thursday.

The blast at the West Fertilizer Company on Wednesday -- which destroyed dozens of homes in the small town of West -- came after a fire at the plant, which is believed to have held large amounts of potentially volatile ammonia.

In 2012 the company was fined $10,100 by the US Pipelines and Hazardous Materials Safety Administration (PHMSA) for mislabeled cargo tanks and "failing to develop and adhere to a security plan" for transporting a large quantity of anhydrous ammonia, according to a copy of the citation obtained by AFP.

The company reached a settlement with US regulators in which it paid a $5,250 fine, the documents show.

The violations concerned the transport of anhydrous (without water) ammonia, and not its storage at the factory itself, which exploded nearly an hour after a fire broke out Wednesday evening, according to local officials.

In 2006, the Texas Commission on Environmental Quality carried out an "odor complaint investigation" at the plant and issued a notice saying it was operating without authorization.

The matter was resolved later that year when the company filed an application, the TCEQ said in a statement.

The fertilizer company could not immediately be reached for comment.

The Dallas Morning News meanwhile reported that the company had told the US Environmental Protection Agency and local authorities that its factory posed no risk of fire or explosion despite holding up to 54,000 pounds of ammonia.

Anhydrous ammonia, NH3, is a pungent, flammable colorless gas commonly used as a soil fertilizer.

It is compressed into liquid form and stored in high-pressure tanks. It becomes potentially explosive when high concentrations are ignited by a heat source greater than 1,204 F (651 C).

Officials have not yet confirmed the cause of the explosion on Wednesday -- which registered as a 2.1-magnitude seismic event and sent up a massive fireball and plume of smoke -- and said they have not ruled out foul play.

An alternative form of ammonia delivery is in granulated form as ammonium nitrate, or NH4NO3, which derives from NH3 combined with nitric acid.

Ammonium nitrate has been found in primitive fertilizer bombs of the kind used in the 1995 bomb attack on a US federal building in Oklahoma City, and in Oslo in 2011 by Norwegian mass murderer Anders Breivik.

Accidents involving ammonium nitrate include a 2001 blast at a French plant where 300 tonnes of the chemical were stored, which killed 31 people. The exact cause of that disaster has never been determined.