The actual cause of the fire and subsequent blast at the West Fertilizer Co 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 Bureau of Alcohol, Tobacco, Firearms and Explosives.

Investigators had hoped to determine by May 10 what caused the explosion, Texas Fire Marshal Chris Connealy said at a state legislative hearing last week. But the fire marshal’s office said this week that the investigation will take one or two weeks beyond that.

“We’re not going to leave anything unturned on the scene, and there just happened to be more that needed to be turned,” Assistant State Fire Marshal Kelly Kistner said on Tuesday.

The blast caused an estimated $100 million in damages to homes, businesses and schools near the fertilizer plant. The dead included 11 firefighters and other first responders who had rushed to contain a fire at the plant.

Ammonium nitrate is a dry fertilizer mixed with other fertilizers 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 1995 bombing of a federal building in Oklahoma City that left 168 people dead.

Anhydrous ammonia, another fertilizer 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. Investigators believe the fire started somewhere in the 12,000 square foot (1,100 square meter) fertilizer and seed building. They continue to work on pinpointing the exact location.

“The number-one issue that we’re trying to resolve right now is, where did the fire start and how did the fire start,” Kistner said.

Investigators said they have eliminated the following causes for the initial fire: weather, natural causes, anhydrous ammonia, a 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 fertilizer 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. DM

Photo: The remains of a fertilizer plant burn after an explosion at the plant in the town of West, near Waco, Texas early April 18, 2013. The deadly explosion ripped through the fertilizer plant late on Wednesday, injuring more than 100 people, leveling dozens of homes and damaging other buildings including a school and nursing home, authorities said. REUTERS/Mike Stone