A criminal investigation has been launched into the massive Texas fertiliser plant explosion that killed 14 people, after weeks of largely treating the blast as an industrial accident.
The announcement came on the same day US government agents said they found bomb-making materials belonging to a paramedic who helped evacuate residents on the night of the explosion.
Bryce Reed was arrested for possessing a destructive device, but law enforcement officials said they had not linked the charge to the April 17 fire and blast at West Fertiliser Company, near Waco.
"It is important to emphasise that at this point, no evidence has been uncovered to indicate any connection to the events surrounding the fire and subsequent explosion ... and the arrest of Bryce Reed by the ATF," the McLennan County Sheriff's Office said.
The agencies will join the State Fire Marshall's Office and the Bureau of Alcohol, Tobacco, Firearms and Explosives, which have been leading the investigation and never ruled out that a crime may have been committed.
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 give any further reasons for the criminal investigation and said no additional information would be released.
Reed is in custody and a criminal complaint said he was arrested after McLennan County deputies were called earlier this week to a home in Abbott, a town about five miles from West, and found bomb-making materials - including a galvanised metal pipe, canisters filled with fuses, a lighter, a digital scale and a variety of chemical powders. "After further investigation, it was determined that the resident had unwittingly taken possession of the components from Reed on April 26," said the complaint signed by ATF special agent Douglas Kunze. Reed made an initial appearance in federal court in Waco, but did not enter a plea.
An ATF explosives specialist and a chemist examined the items and agreed the "combination of parts can be readily assembled into a destructive device", the complaint says.
Officials have largely treated the West explosion as an industrial accident, though investigators 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.