The State Fire Marshal’s Office has determined ammonium nitrate stored at the West Fertilizer Plant is what exploded last month. But investigators are still trying to figure out what caused the fire that led to that explosion.
Alone, anhydrous ammonia and ammonium nitrate are not known as flammable substances. But both - and ammonium nitrate in particular - may be very dangerous when mixed with other chemicals, such as diesel fuel or oil.
When mixed, ammonium nitrate may be used to create explosives, as it was in the 1995 Oklahoma City bombing.
Investigators say they know the fire wasn’t caused by weather or other natural causes. They've also determined water used to fight the fire did not contribute to the blast.
The investigation into the plant explosion was supposed to be complete by this Friday but officials now say it could take another week or two. Fifteen people were killed and hundreds injured in the April 17 blast.