Federal and state investigators have ruled the cause of the deadly fire and explosion at a fertilizer facility in West as undetermined.
The April 17 explosion at the West Fertilizer Co. killed 15 and injured hundreds while laying waste to much of the tiny town.
Texas State Fire Marshal Chris Connealy said a ruling of undetermined is made when the cause "cannot be proven to an acceptable level of certainty."
"This could be due to insufficient information or if multiple causes could not be eliminated," he said.
Robert Champion, Bureau of Alcohol, Tobacco, Firearms and Explosives Dallas Field Division agent in charge, said the following could not be eliminated: a 120-volt electrical system, a golf cart and an intentionally set fire.
A West first responder was arrested last week on suspicion of possessing bomb-making materials, but Champion said investigators would not speculate about whether Bryce Reed is connected to the blast.
Investigators have ruled several possible causes, such as the rekindling of a fire, spontaneous ignition, the 480-volt electrical system that ran the plant's heavy equipment, anhydrous ammonia, ammonia nitrate, smoking and weather.
The investigation into the fire is open and ongoing, although the excavation of the scene is complete. The State Fire Marshal's Office, the ATF, the Department of Public Safety, the McLennan County Sheriff's and District Attorney's Offices will continue to have staff in West to work on the investigation.
Senators John Cornyn and Ted Cruz released the following statement on the investigation:
"We appreciate the individuals and agencies who have worked to investigate the cause of the tragic explosion in West. Our prayers remain with those struggling to recover and mourning the loss of loved ones. While the cause remains undetermined, it is our sincere hope that at the end of the investigation, the residents of West can find closure and begin to heal."
Details of the fire
The fire, which started in the fertilizer and seed building, sparked two explosions, investigators said. The explosions were "milliseconds" apart, and most people would have registered them as one explosion, investigators said.
The fire was reported at 7:29 p.m., and the two explosions occurred at approximately 7:51 p.m. The fire department was dispatched at 7:32 p.m. and firefighters arrived at 7:38 p.m.
Water from the firefighting activities did not contribute to the cause of the explosion.
The blast left a 37-block area of damage and a crater that was 93 feet wide and 10 feet deep. The furthest piece of evidence was found two and half miles away, but most evidence was found within a 3,000-foot radius.
The origin of the fire was determined to be in the northern area of the fertilizer and seed building, in what is referred to as the seed room.
The golf cart that could not be eliminated as a possible cause of the fire was located in that room. The 120-volt electrical system that could not be ruled out was also located in the building.
Investigators estimate that between 28 and 34 tons of ammonium nitrate exploded, the equivalent of approximately 15,000 to 20,000 pounds of TNT.
An additional 20 to 30 tons in the building and approximately 100 tons of ammonium nitrate in a nearby railcar did not explode.
Connealy said the families of those killed in the blast were briefed on the findings before the press conference.
In addition to the investigation into the origin and cause of the fire, the fire marshal's office has also been conducting a firefighter line of duty death investigation, as required by state law. Work on the report for that investigation is expected to take several months.
The Texas Rangers and McLennan County Sheriff's Office last week opened a criminal investigation into the blast. The fire marshal's office said Thursday that the fire investigation had been considered a criminal investigation since the case was opened.