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Explosions tore through a fuel depot north of London before dawn on Sunday, creating a huge tower of smoke and flame and seriously injuring four people in what police said appeared to be an accident.

"All indications at this stage are that this was an accident. However clearly we will keep an open mind," Hertfordshire Chief Constable Frank Whiteley told a news conference after Britons, still on edge from July attacks in London, awoke to images of rampaging fire and destruction.

The Buncefield depot supplies petrol and fuel oils for a large part of southeast England. Oil is brought to the depot, near the town of Hemel Hempstead, in an underground pipeline from tankers unloading on Britain's east coast.

Whiteley said there were around 36 casualties, about four of whom suffered serious injuries.