(CNN) -- Investigators have not ruled out an intentional fire being behind explosions at a fertilizer plant in the small town of West that left 15 people dead, the Texas fire marshal said Thursday.
State Fire Marshal Chris Connealy said investigators were unable to rule out three possible causes, including a spark from a golf cart, an electrical short or an intentionally set fire.
"The cause cannot be proven to an acceptable level," Connealy told reporters.
Investigators said the incident was actually two simultaneous blasts triggered by the fire. The blasts, which registered on seismographs as a magnitude 2.1 earthquake and was felt 50 miles away, caused damage to a 37-block area of the town.
The announcement follows news last week that authorities launched a criminal investigation into the April 17 fire and explosion in West, about 70 miles southwest of Dallas.
Authorities announced the criminal investigation last Friday, the same day investigators said a paramedic who responded to the fire was arrested on suspicion of possession of a destructive device after investigators allegedly found materials to make a pipe bomb at his home.
Federal authorities have not said whether the arrest of Bryce Reed was connected to the fire and blast, and Robert Champion of the Dallas office of the Bureau of Alcohol, Tobacco, Firearms and Explosives declined Thursday to discuss whether a pipe bomb could cause the damage that led to the explosions.
The state fire marshal's office had previously ruled out four potential causes: weather, natural causes, anhydrous ammonium and ammonium nitrate in a rail car.
The powerful explosion leveled a portion of the town, damaging numerous homes, a nursing home and the town's high school and middle school.
In that weeks that followed, scores of investigators have following up on leads. At least 60 have been on site each day and have conducted more than 400 interviews in trying to determine how the fire started and what caused the explosion, authorities said.
The West Fertilizer Co., which operated the facility, had been cited by federal regulators twice since 2006.
In 2012, the Transportation Department's Pipeline and Hazardous Materials Safety Administration fined West Fertilizer $5,250 for storing anhydrous ammonia in tanks that lacked the proper warning labels. The agency originally recommended a $10,000 penalty, but it was reduced after the company took corrective action.
In 2006, the EPA fined it $2,300 and told the owners to correct problems that included a failure to file a risk management program plan on time. The Texas Commission on Environmental Quality also investigated a complaint about the lingering smell of ammonia around the plant the same year.
CNN's Greg Botelho contributed to this report.