According to news reports:
The massive blast witnessed on Tuesday was likely caused by the detonation of more than 2,700 tons of ammonium nitrate that had been stored in a warehouse at the dock in Beirut’s port ever since it was confiscated from a cargo ship in 2014, Lebanese Interior Minister Mohammed Fahmi told local media.
Though the blast remains under investigation, Lebanese officials believe that a fire engulfed what initially appeared to be fireworks at a warehouse – igniting the explosion that leveled most of the port, blew out windows and collapsed balconies and roofs miles from its epicenter and sent out a shockwave heard from as far away as Cyprus. What initially started the fire at the port remains unclear.
Online videos of the disaster's initial moments show sparks and lights inside the smoke rising from the blaze, just prior to the massive blast. The white cloud that accompanied the explosion appeared to be a condensation cloud, often common in massive explosions in humid conditions that can follow the shock waves of an explosion, experts told the Associated Press. Orange clouds also followed the blast, likely from toxic nitrogen dioxide gas that's released after an explosion involving nitrates.
Source: Beirut explosion: Rescue workers frantically search for survivors as Lebanon capital confronts massive devastation
Based on the timeline and the size of the cargo, that ship could be the MV Rhosus. The ship was initially seized in Beirut in 2013 when it entered the port due to technical problems, according to lawyers involved in the case. It came from the nation of Georgia, and had been bound for Mozambique.
“Owing to the risks associated with retaining the ammonium nitrate on board the vessel, the port authorities discharged the cargo onto the port’s warehouses," the lawyers wrote in a 2015 article published by shiparrested.com. “The vessel and cargo remain to date in port awaiting auctioning and/or proper disposal.”
“Before the big explosion, you can see in the center of the fire, you can see sparks, you can hear sounds like popcorn and you can hear whistles," Hayoun told The Associated Press. "This is very specific behavior of fireworks, the visuals, the sounds and the transformation from a slow burn to a massive explosion.”
“It looks like an accident,” Lewis told the AP. “First, there was a fire preceding the explosion, which is not an attack. And some of the videos show munitions what I could call popcorning, exploding like ’pop, pop, pop, pop.’”
He added that “it’s very common to see fires detonate explosives."
“If you have a fire raging next to something explosive, and you don’t put it out, it blows up," he said.
Source: Fireworks, ammonium nitrate likely fueled Beirut explosion
According to the NYT:
Some 2,750 tons of ammonium nitrate unloaded from a disabled vessel in 2014 had been stored in a port warehouse. Then yesterday, a welding accident ignited nearby fireworks — which caused the ammonium nitrate to explode.
IMPACT: The consequences of yesterday’s explosion will be even more serious than the immediate casualties and property damage. The main grain silo, which holds some 85 percent of the country’s cereals, was destroyed. Even more, the port will no longer be able to receive goods. Lebanon imports 80 percent of what it consumes, including 90 percent of its wheat, which is used to make the bread that is the staple of most people’s diets. About 60 percent of those imports come through the port of Beirut. Or, at least, they did.
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