(CNN) - Texas authorities are launching a criminal investigation into last month's deadly fertilizer distribution facility explosion in West, Texas, the Texas Department of Public Safety said Friday.
"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," DPS Director Steven McCraw said.
The April 17 fire and explosion at West Fertilizer Co. killed 14 people and devastated the small town of West.
On Monday, the state fire marshal's office said it ruled out four potential causes: weather, natural causes, anhydrous ammonium, and ammonium nitrate in a rail car.
The fire began in the fertilizer and seed building, but authorities still were trying to figure out the exact spot, the fire marshal's office said Monday.
The blast happened about 20 minutes after the first report of a fire at the fertilizer facility. It registered on seismographs as a magnitude-2.1 earthquake and could be felt 50 miles away.
The explosion damaged numerous homes, a nursing home and the town's high school and middle school, all of which were built within a few hundred feet of the plant. West is about 70 miles south-southwest of Dallas,
Scores of investigators have followed up on scores of leads relating to the explosion. At least 60 investigators have been on site each day and have conducted 411 interviews in trying to determine how the fire started and what caused the explosion.
"The citizens of McLennan County and Texas must have confidence that this incident has been looked at from every angle and professionally handled -- they deserve nothing less," McLennan Sheriff Parnell McNamara said.
West Fertilizer Co., which operated the facility, had been cited by federal regulators twice since 2006.
In 2012, the Transportation Department's Pipeline and Hazardous Materials Safety Administration fined West Fertilizer $5,250 for storing anhydrous ammonia in tanks that lacked the proper warning labels. The agency originally recommended a $10,000 penalty, but it was reduced after the company took corrective action.
In 2006, the EPA fined it $2,300 and told the owners to correct problems that included a failure to file a risk management program plan on time. The Texas Commission on Environmental Quality also investigated a complaint about the lingering smell of ammonia around the plant the same year.