Experts say the world may never know exactly what triggered a massive explosion that shook Lebanon on Tuesday, killing at least 135 people and injuring thousands more.

But immediate video evidence suggests the chain of events began with a fire – rather than a terrorist or military attack – detonating tons of stored ammonium nitrate and obliterating a portion of Beirut.

The explosion had the force of at least 500 tons of TNT, according to a U.S. government source who was not authorized to speak publicly. The estimate was based on the widespread destruction, said the source, who has experience with military explosives.

The blast caused carnage over a 6-mile radius and was felt more than 100 miles away.

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