WEST, Texas (AP) - Investigators have completed their scene investigation but not ruled out criminal activity as the cause of a massive explosion at a fertiliser plant that killed 15 people and flattened part of a tiny Texas town, officials said on Thursday.

The April 17 blast at West Fertilizer injured 200 and levelled part of the tiny town of West. Officials have spent one month combing through debris and speaking to hundreds of witnesses.

"At this time, the state fire marshal's office and ATF are ruling the cause of this fire is undetermined," state fire marshal Chris Connealy said at a news conference on Thursday. A criminal investigation continues.

Possible causes of the fire that triggered two explosions have been narrowed to a 120-volt electrical system at the plant, a golf cart or an intentionally set fire, officials said.

The golf cart was parked in the seed room and had been recalled by its manufacturer. All that was found of it were a brake pad and an axle.

"There's a history of golf carts actually starting fires," said Mr Brian Hoback, national response team supervisor for the Bureau of Alcohol, Tobacco, Firearms and Explosives. The batteries hold a charge and when they fail they can ignite the materials around them.

Assistant state fire marshal Kelly Kistner said investigators estimated that between 28 tons and 34 tons of ammonium nitrate on the site exploded. But there were about 150 tons of the chemical on the site at the time, including 100 tons in a rail car that did not explode. The chemical that exploded was stored in wooden bins. He said the ammonium nitrate was the equivalent of 15,000 to 20,000 pounds of dynamite.

Investigators had ruled out other possible causes, including smoking or a weather-related fire.

Officials have determined that ammonium nitrate exploded, but they do not know what started the initial fire. The fire created the conditions for an initial smaller explosion, which Mr Kistner said was only "milliseconds" before the larger explosion.

Mr Bryce Reed, a paramedic who responded to the blast, pleaded not guilty on Wednesday to a charge he possessed bomb-making materials, but authorities have stressed they have nothing linking Mr Reed to the blast. Federal investigators allege Mr Reed had materials for a pipe bomb that he gave to someone else.

The dead included 10 first responders and two volunteers trying to fight the initial fire, which was reported 18 minutes before the blast.

Ms Rachel Moreno, a spokesman for the Texas State Fire Marshal's Office, said the death toll had officially reached 15 with the determination by a local justice of the peace that an elderly man who died after being evacuated from the nursing home had been an explosion-related death. The nursing home's medical director previously had said the man died of his pre-existing ailments.

Two months before the explosion, the plant reported it had the capacity to store as much as 270 tons of ammonium nitrate, though how much was actually on site when the blast occurred is unknown.

Ammonium nitrate is a chemical used as a fertilizer that also can be used as a cheap alternative to dynamite. It was the chemical used in the 1995 Oklahoma City bombing.

The fire marshal's office had previously ruled out several possible causes for the initial fire, including another fertiliser stored on site, anhydrous ammonia; a rail car on the site that was carrying ammonium nitrate; and a fire within a storage bin of ammonium nitrate.

Mr Daniel Keeney, a spokesman for Adair Grain Co, which owned and operated West Fertilizer, has said the company is cooperating with authorities, but declined to comment further.