Investigators have confirmed that ammonium nitrate was the trigger for the explosion at a West, Texas, fertiliser plant last month that left 14 people dead and some 200 injured, according to the Texas state fire marshal's office.
The actual cause of the fire and subsequent blast at the West Fertilizer facility is still being determined, investigators said.
The fire marshal's office has been leading the investigation of the April 17 blast, along with the federal Alcohol, Tobacco, Firearms and Explosives agency (ATF).
The blast caused an estimated US$100 million in damages to homes, businesses and schools near the fertiliser plant, and killed several firefighters and other first responders who rushed to the scene of a fire at the fertilizer plant.
Ammonium nitrate is a dry fertiliser mixed with other fertilisers such as phosphate and applied to crops to promote growth. It can be combustible under certain conditions, and was used as an ingredient in the Oklahoma City bombing in 1995 that left 168 people dead.
Anhydrous ammonia, another fertiliser component, was also stored on site at the West Fertilizer facility and there was some early speculation that it may have been the source of the explosion.
More than 70 investigators have developed over 200 leads, from which over 400 interviews have been conducted. Thus far, investigators do know the origin of the fire was in the fertiliser and seed building. The investigators continue to work on pinpointing an exact location of the fire's origin within the building that is over 12,000 square feet (1100 square metres).
Investigators said they have eliminated the following causes for the initial fire: weather, natural causes, anhydrous ammonium, the railcar containing ammonium nitrate, and a fire within the ammonium nitrate bin.
Additionally, they said water used during fire fighting activities did not contribute to the cause of the explosion as some had speculated.
Even though the investigation into the cause has not been determined, at least seven lawsuits have so far been filed against Adair Grain Inc, which owned the fertiliser facility.
Plaintiffs claim negligence by the plant employees and are seeking millions of dollars in claims. Four insurance companies are among those suing Adair Grain seeking to recover claims they are paying to individuals and businesses hurt in the explosion.