HOUSTON - US authorities said Thursday that investigators have not ruled out the possibility that last month's fatal blasts at a fertilizer plant in the US state of Texas were caused by an intentionally-set fire.

Meanwhile, officials said at a news briefing Thursday that the cause of the two explosions at the plant that killed 15 people, wounded about 200 and leveled part of the Texas town of West remains undetermined, according to NBC news.

Texas Fire Marshal Chris Connealy promised to "leave no stone unturned to make sure everything is done".?He said that 30 different local, state and federal agencies were working "with one common goal: to understand what happened so we can give closure to these families."

Investigators said the fire began in a fertilizer and seed building called the seed room. The possible causes of the disaster included arson, a failure of one of the plant's two electrical systems and a compromised battery on a golf cart, investigators said.

Other possible triggers, including the weather, some sort of spontaneous ignition, failure of the facility's second electrical system, two ammonium compounds used in the fertilizer-making process and smoking, had been ruled out, according to the report.

Investigators said the fire caused at least 28 tons of ammonium nitrate, a highly combustible powder, to explode in the seed room. Fortunately, a rail car sitting outside holding about 100 more tons of the compound did not blow up, they said.

Bryce Reed, a paramedic who was among the first responders to the blast, pleaded not guilty Wednesday to charges that he possessed bomb-making materials. But officials said Thursday that they have nothing linking Reed to the explosions.

Investigators refused to go into other details of their investigation, which was being handled as a criminal matter. They said the investigation was expected to take several more months.