In a surprising turn in the investigation, one of the first responders, a paramedic, is in custody facing explosives charges.
West resident Bryce Reed has been arrested and will be appearing in a Texas court room sometime Friday.
Authorities say they found a pipe bomb in his possession and he will be charged with possession of a destructive device.
Reed is well known in the town of West, and in the wake of the explosion at the fertilizer plant, he did a number of television interviews with members of the press.
Now, it is still not clear if Reed is being linked to the plant explosion, but sources tell ABC News he will be getting a closer look because he was at the scene that night.
The plant exploded on the night of April 17 after a fire erupted at the facility in the rural town. In addition to the fatalities, a large part of the small town was damaged in the explosion.
The facility was producing and storing ammonium nitrate fertilizer - the same type of chemical that was used in the Oklahoma City bombing.
Until now, the blast has been largely treated as an industrial accident. Friday, the Texas Department of Public Safety said in a statement that the agency has instructed the Texas Rangers and the McLennan County Sheriff's Department to conduct a criminal probe.
Authorities are not providing details on what prompted the probe and are not connecting Reed's arrest to that criminal investigation.
More information was expected to be released following Reed's court appearance.
ABC News contributed to this report.