I feel safe saying this was not terrorism, this was an accident plain and simple. The facility was already on fire before it exploded and the local authorities knew it was going to explode shortly before it did which is why they were busy evacuating the area.
They just didn’t know it would explode so quickly, and couldn’t notify enough people with enough time. The heros were probably the firefighters who stayed behind in the area trying to fight to fire. They no doubt bought (with their lives) many people time to evacuate to a safer distance.