(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.
Forensic mappers work the crater at the site of the fire and explosion in West, Texas, on Wednesday, April 24. The plant run by West Fertilizer Co. in the small Texas town exploded on Wednesday, April 17, killing 14 people, most of them emergency responders. Dozens were injured.
Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives on April 24 search the bank of rail tracks for evidence at the site of the explosion.
Smoke billows into the sky immediately after the explosion at West Fertilizer Co. in West, Texas, on Wednesday, April 17, as captured by CNN iReporter Brian Kitchen. The deadly explosion damaged buildings for blocks in every direction.
The West Fertilizer Co. lies in ruins in West, Texas, on Thursday, April 18, the day after the accident.
The remains of an apartment complex lie on cars on April 18.
A deer head mount sits inside a car parked next to a apartment complex damaged in the explosion.
Search and rescue workers comb through what remains of a 50-unit apartment building, in foreground, and a nursing home on April 18.
The remains of a car sit in front of an apartment complex destroyed after the fertilizer plant blast.
A Valley Mills Fire Department firefighter walks the remains of an apartment complex next to the fertilizer plant on April 18.
Fire personnel check out the railroad tracks near the fertilizer plant on April 18.
Valley Mills Fire Department personnel view the railroad tracks near the fertilizer plant on April 18.
Smoke rises April 18 from the rubble of a house next to the fertilizer plant.
A railroad boxcar lies on its side near the plant on April 18.
A Texas State Trooper stops people from entering a neighborhood near the plant on April 18.
A chimney is the only part of a home left standing after Wednesday’s explosion.
Search and rescue workers comb through what remains of a 50-unit apartment building on April 18.
Workers clean up shattered windows at a store in West, Texas, on April 18.
Debris litters the fields around a fertilizer plant on April 18, the day after the explosion. The blast damaged 50 to 60 homes in a five-block area, officials said.
This satellite image shows West, Texas, as captured on January 30, 2012. The fertilizer plant is on the right center of the photo, just northeast of the oval track of the town’s middle school.
A sheriff’s deputy comforts a woman at a command post on April 18.
Brandon Smith removes broken glass from the West Thrift Shop on Thursday.
Workers board up a furniture shop with shattered windows on Thursday.
Remains of a fertilizer plant burn in West early April 18.
Water is sprayed at the burning remains of the plant.
The deadly explosion leveled dozens of homes and damaged other buildings on April 18, including a school and nursing home, authorities said.
A vehicle is seen near the remains of the fertilizer plant on April 18.
With smoke rising in the distance, a law enforcement officer runs a checkpoint at the perimeter about half a mile from the plant on April 18.
Waco Police spokesman William Swanton speaks to reporters about the explosion on April 18.
Glass from blown-out windows lies shattered on the sidewalk and street after the blast on April 18.
Earth-moving equipment rolls through the downtown area in the middle of the night on April 18.
Shattered glass covers items in the front of a thrift store on April 18.
Photos: Texas fertilizer plant explodes
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.