Texas authorities have launched a criminal probe into the deadly explosion at West Fertilizer Company on April 17.
The disaster “severely impacted” the community in the town of West, a law enforcement official said.
The blast at West Fertilizer Company killed 14 people, wounded 200, and caused a tremor as powerful as a small earthquake.
Meanwhile, paramedic Bryce Reed, who responded to the blast, was charged with possessing pipe bomb components, prosecutors said.
The explosion flattened homes, shattered a block of flats and badly damaged a nursing home and several schools.
“This disaster has severely impacted the community of West, and we want to ensure that no stone goes unturned and that all the facts related to this incident are uncovered,” Texas Public Safety Director Steven McGraw said.
McLennan County Sheriff Parnell McNamara said: “The citizens of McLennan County and Texas must have confidence that this incident has been looked at from every angle.”
The statement did not detail any further reasons for the criminal investigation and said no additional information would be released.
Authorities had said earlier there was no indication that the explosion and the fires that preceded it were anything other than an industrial accident.
Also on Friday, paramedic Bryce Reed made a brief court appearance in the town of Waco where he was charged with owning an unregistered destructive device.
According to the criminal complaint, police were called to a Texas residence on Tuesday where they discovered bomb-making components including a galvanized metal pipe, fuse, lighter and explosive powder in bags.
A Bureau of Alcohol, Tobacco, Firearms and 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.
Law enforcement officials have not linked the charge to the fire or the explosion at the fertilizer plant.
Sheriff Parnell McNamara said in a statement: “It is important to emphasize 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.”
Bryce Reed, who helped in the emergency response to the blast, did not enter a plea.
If convicted, Bryce Reed could face up to 10 years in prison and be fined up to $250,000.
Bryce Reed told the Dallas Morning News he had assumed radio command of the response to the fertilizer plant incident after it killed his superiors and colleagues.