Story highlights "The cause cannot be proven to an acceptable level," the state fire marshal says

Investigators have not ruled out an intentionally set fire, the official says

Authorities also have not ruled out whether a spark from a golf cart caused the fire

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.

Photos: Photos: Texas fertilizer plant explodes Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Forensic mappers work the crater at the site of a fire and explosion in West, Texas, on April 24, 2013. The West Fertilizer Co. plant in the small Texas town exploded days earlier on April 17, killing 15 people. Hide Caption 1 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives search the bank of rail tracks for evidence at the site of the explosion. Hide Caption 2 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Search and rescue workers comb through what remains of a 50-unit apartment building, in foreground, and a nursing home on April 18, 2013. Hide Caption 3 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – The remains of an apartment complex lie on cars. Hide Caption 4 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – A deer head mount sits inside a car parked next to an apartment complex damaged in the explosion. Hide Caption 5 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – A Valley Mills Fire Department firefighter walks through the remains of an apartment complex next to the fertilizer plant. Hide Caption 6 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Smoke rises from the rubble of a house next to the fertilizer plant. Hide Caption 7 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – A railroad boxcar lies on its side near the plant. Hide Caption 8 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – A Texas State Trooper stops people from entering a neighborhood near the plant. Hide Caption 9 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – A chimney is the only part of a home left standing after the explosion. Hide Caption 10 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Brandon Smith removes broken glass from the West Thrift Shop. Hide Caption 11 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Search and rescue workers comb through what remains of a 50-unit apartment building. Hide Caption 12 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – The West Fertilizer Co. lies in ruins. Hide Caption 13 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – A sheriff's deputy comforts a woman at a command post. Hide Caption 14 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Remains of the fertilizer plant burn in the early morning after the explosion. Hide Caption 15 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Water is sprayed on the remains of the plant. Hide Caption 16 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – With smoke rising in the distance, a law enforcement officer runs a checkpoint at the perimeter about half a mile from the plant. Hide Caption 17 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Waco Police spokesman William Swanton speaks to reporters about the explosion. Hide Caption 18 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Glass from blown-out windows lies shattered on the sidewalk and street after the blast. Hide Caption 19 of 20 Photos: Photos: Texas fertilizer plant explodes Texas fertilizer plant explodes – Shattered glass covers items in front of a thrift store. Hide Caption 20 of 20

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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.