CCTV said a shipment of explosives had detonated but this was not confirmed.

The first explosion at about 23:30 (15:30 GMT) on Wednesday was followed by another seconds later. Shockwaves were felt several kilometres away.

The editor of the BBC's Chinese Service, Raymond Li, says all indications are that it was an industrial accident.

The China Earthquake Networks Centre said the magnitude of the first explosion was the equivalent of detonating three tons of TNT, while the second was the equivalent of 21 tons of the explosive.

The Blue Plains Wastewater Treatment Facility sits on the Potomac River in southwest Washington, D.C. Like most facilities before 9-11, Blue Plains used chlorine gas and other hazardous compounds in its treatment process. In fact, when the plane hit the Pentagon on 9-11, Blue Plains had six 90-ton railcars on site storing dangerous chemicals just four miles away from the Capitol. Three were filled with chlorine gas, one was filled with sulfur dioxide, and two railcars were being unloaded; each filled with chlorine gas and sulfur dioxide.

Not only were these chemicals a threat when in storage on the site, but they also created a hazard in transit as they were hauled by rail through downtown D.C. on their way to Blue Plains. To get a sense of the exposure they represented, consider that in January 2005, when a freight train pulling three tankers full of liquidized chlorine and one tanker of sodium hydroxide slammed into a parked train in Graniteville, South Carolina, it released 11,500 gallons of chlorine gas. Nine people died and at least 529 were injured. That was without any malicious intent and in a rural location.

Chlorine gas was infamous during trench warfare in World War I. It irritates the eyes, nose, and skin in small amounts, but in concentrated form, the yellowish green gas causes the lungs to fill with liquid, drowning the victims to death.