At around 6:50PM yesterday in West, Texas, just a few miles from Waco, a fertilizer plant plant owned by Adar Grain Inc. exploded with a force that reduced 50 units at a nearby apartment complex to "just a skeleton," registering as the equivalent of a magnitude 2.1 earthquake with the US Geological Survey.

About 50 minutes earlier, a West Fertilizer building called the "dry barn" caught fire and was completely engulfed in flames at the time the explosion occurred. The dry barn, said a West employee, was used to store ammonium nitrate — a chemical compound of ammonia and nitric acid that is both a potent fertilizer and, under the right circumstances, a powerful explosive. It’s used in everything from construction and mining to IEDs, but probably best known as the fertilizer in the bomb that Timothy McVeigh used in his 1995 Oklahoma City attack. Last night, Department of Public Safety spokesman DL Wilson compared the current scene in West to the damage sustained by McVeigh’s target, the Alfred P. Murrah Federal Building. At this point, over 100 people are reported to have been injured in the blast, with at least five believed dead.

"Water, under certain conditions, will set off anhydrous ammonia."

It’s still unclear whether ammonium nitrate, other chemicals, or a combination, were responsible for the blast at West Fertilizer. Many media reports so far have focused on the presence of anhydrous ammonia, one of the components used to make ammonium nitrate. Physicist Michio Kaku told CBS News that water from fire hoses could have reacted with the anhydrous ammonia on the premises. "Water, under certain conditions, will set off anhydrous ammonia," he explained. "The hose water might have set off the anhydrous ammonia, creating a chain reaction." Under normal conditions, ammonia is not explosive.

Hot pools of ammonium nitrate can form when it's confined

Likewise, ammonium nitrate is rated as non-flammable and since it doesn’t detonate under normal conditions (i.e., when it’s not in the presence of fuel oil), it’s generally regarded to be safe. However, an Australian government report notes that in a fire, hot pools of ammonium nitrate can form when the chemical is confined, making it very sensitive to shock if it comes in contact with other flammable or combustible materials, oil, metals, or other chemicals. It is also a strong oxidizing agent, supplying oxygen to the fuel in a fire even when there’s no air available. That strong oxidizing property can even risk causing combustible materials like paper and oil to ignite.

There's a lengthy history of deadly, accidental ammonium nitrate explosions, including the Texas City disaster of 1947, when a cargo ship being loaded with ammonium nitrate caught fire and exploded, killing at least 576 and leveling much of the town, still considered the deadliest industrial accident in US history. But there have been at least 17 major fatal ammonium nitrate explosions around the globe since 1921, as The Guardian reports. Many have occurred overseas, including in France and Australia. A University College of London organic chemistry professor just happened to be nearby one of these such explosions in Toulouse, France, and recalled the incident at Chemistry World magazine, saying: "I had previously heard three terrorist explosions in London, UK. I had also witnessed some pretty substantial bangs during my time as a graduate student or while doing lecture demonstrations. But this was different. This was big."

For the people of West, Texas, the story is still unfolding. Rescue teams are still searching for the missing, and Police Sgt. William Patrick Swanton is telling the press that it will be a slow process going forward. "It is tedious, it is time consuming, it is a very methodical process they are doing."

Carl Franzen contributed to this report.