KeithBinns/iStockBy IBTISSEM GUENFOUD and GUY DAVIES, ABC News

(PARIS and LONDON) -- An explosion at a warehouse stocked with 2,750 tons of ammonium nitrate in the Beirut port changed the face of a city almost instantly.

The exact cause of the Beirut blast is still under investigation, but the catastrophic effects of ammonium nitrate were there for all to see: A massive explosion causing shock waves throughout the city and a mushroom cloud of smoke billowing high into the sky.

So far, at least 137 are confirmed dead, including one American, and over 5.000 were injured in the blast, according to the Lebanese authorities.

But, according to experts, ammonium nitrate is not itself classified as dangerous, and requires a lot of energy to ignite.

"[Ammonium nitrate] is a great fertilizer for growing plants, but it's also wonderful as an explosive," Jimmy Oxley, a chemistry professor at the University of Rhode Island, told ABC News. "200 factories make ammonium nitrate around the world… They are mining all over."

Ammonium nitrate was involved in the 1947 Texas City disaster, one of the largest industrial explosions in history.

But the chemical has a more nefarious use -- as an explosive. The chemical compound was used to make explosives for the Oklahoma City bombing that killed 168 people 25 years ago. The fertilizer and fuel bomb used then contained just over two tons of ammonium nitrate to carry out the deadliest domestic terror attack on U.S. soil. The warehouse in Beirut is believed to have carried more than 1,000 times that amount.

Yet, Oxley, who carried out research into the chemical compound, said that while the ammonium nitrate has been used as an explosive, the compound cannot be solely responsible for the explosion.

"The product itself is not classified as dangerous," said Gilles Choquet, president of AIS service, a training organization in risk prevention in explosive environments. "It is the storage of this product that could generate risks, with the rise in temperatures, which will be all the greater when there is so much product."

Port officials have been placed on house arrest pending the results of the investigation on the suspected gross negligence of leaving the over 2,500 tons in the port's warehouse for six years, Lebanon President Michel Aoun announced.

The ammonium nitrate appears to have been confiscated from a commercial cargo ship abandoned in Beirut in 2013, allegedly with a Russian owner, and then confiscated by the Lebanese authorities a year later; it's origins linked to former Soviet republic, Georgia.

"I'm not surprised it's coming from Georgia or Russia," Oxley said. "Cheap natural gas [is] there, and natural gas is one of the precursors for making ammonium nitrate."

The chemical, however, requires a lot of energy to become explosive, and is not, in itself, highly combustible.

"What is certain is that when this product is on its own, it cannot explode," Choquet said. At the moment, it is "hard to say the exact cause for the explosion," he said, and the authorities may never find one.

The earliest recorded accident involving ammonium nitrate occurred in Oppau in Germany in 1921.

"The man was blasting it with dynamite," said Oxley about the Oppau explosion. "He got away with that several times. One day he decided to do a bunch of dynamite at once, 600 people died and he was one of them. He put a shock into the ammonium nitrate ... in [Beirut's] situation, if there was a pre-shock into the ammonium nitrate, that would explain what was going on."

According to Oxley, this pre-shock could have been the fireworks; speculated as the cause early on.

"If they got on fire and started a large conflagration, it could come from that," Oxley said. "Ammonium nitrate mixed with fuel will just sit there and look at you ... [to become explosive] it has to have a large input of energy, either from a large fire or a shock wave."

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