Ammonium Nitrate is an oxidizer, it still needs a fuel (diesel/fuel oil/kerosene) to be an explosive. It's not impossible to make it explode without oil, it means that it has to be in an area with a lot of organics.
This isn't the first time ammonium nitrate has been involved in a deadly catastrophe. The deadliest industrial accident in U.S. history occurred in the port of Texas City, Texas, in 1947. A carelessly tossed cigarette started a fire aboard a ship carrying about 2,300 tons (2,086,000 kg) of ammonium nitrate packed in paper sacks. When the chemical exploded, it caused a blast powerful enough to knock people to the ground in Galveston, Texas, 10 miles (16 kilometers) away.
That detonation also caused a chain reaction when a nearby ship, also carrying ammonium nitrate, exploded, setting fires at chemical tanks and oil refineries near the port. An estimated 581 people were killed in the disaster.
According to a report by The Associated Press, experts think that spark came from an initial fire that may have engulfed fireworks being stored at the port.