(CNN) -- Investigators have ruled out four potential causes for the fire that led to an explosion at a fertilizer distribution facility in West, Texas, the state fire marshal's office said Monday in a statement.
Authorities think it was something other than weather, natural causes, anhydrous ammonium or ammonium nitrate in a rail car.
Officials also ruled out water from firefighters as a cause for the massive April 17 blast in which 14 people died.
The fire began in the fertilizer and seed building but authorities are still trying to figure out the exact spot, the statement said.
The investigation into the explosion will last longer than expected, the agency said. The probe was expected to be complete by May 10 but will extend another one to two weeks, the fire marshal's office said.
Scores of investigators have followed up on 237 leads into the explosion. At least 60 investigators have been on site each day and conducted 411 interviews in trying to determine how the fire started and what caused the explosion.
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 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 the company $2,300 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.