Texas police have launched a criminal investigation into a massive fertiliser plant explosion that killed 14 people last month, after weeks of largely treating the blast as an industrial accident in Texas.

A paramedic who helped to evacuate residents the night of the explosion was arrested on a charge of possessing bomb-making materials, including a pipe and chemicals.

It is not clear whether the charge is related to the 17 April blast at West Fertiliser Company.

The Texas Department of Public Safety said that the agency had instructed the Texas Rangers and the McLennan County Sheriff's Department to conduct a criminal inquiry into the explosion.

McLennan County Sheriff Parnell McNamara said residents "must have confidence that this incident has been looked at from every angle and professionally handled -- they deserve nothing less".

The statement did not detail any further reasons for the criminal investigation and said no additional information would be released.

Paramedic Bryce Reed, meanwhile, was in federal custody following his arrest on the charge of possessing a destructive device.

Mr Reed made an initial appearance in federal court, but did not enter a plea.

Bureau of Alcohol, Tobacco, Firearms and Explosives special agent Douglas Kunze says in the complaint that police were called earlier this week to a home where they found a pipe, a variety of chemicals and fuses.

They say the objects belonged to Mr Reed.

Officials have largely treated the explosion as an industrial accident.

Investigators are still searching for the cause of a fire that preceded the blast have said they would treat the area as a crime scene until all possibilities were considered.

Authorities have focused on ammonium nitrate, a chemical commonly used as a fertilizer, but that also can be explosive in the right conditions, as the cause of the explosion.

Mr Reed was one of several paramedics who helped evacuate residents from nearby apartments after the fire erupted and shortly before the explosion.

He said he was devastated by the explosion, which killed one of his closest friends, Cyrus Reed. The two are not related.

Bryce Reed's wife, Brittany Reed, declined to comment early today.

"I can't. No comment, no comment no comment right now," she said before hanging up the phone.