London ? Investigators searched Wednesday for the cause of a massive explosion at one of Britain’s largest fuel depots – including the possibility a tanker driver accidentally sparked the blaze.

They also were looking into whether firefighters had enough equipment and training to deal with such a huge inferno.

The chain reaction of explosions at the Buncefield oil depot early Sunday blackened skies as far away as France and triggered a blaze that burned for days. Forty-three people were injured.

The Fire Brigades Union, which represents most British firefighters, accused fire authorities Wednesday of not having enough foam and not adequately training firefighters to tackle such a blaze.

“It was a catastrophe waiting to happen,” union spokesman Duncan Milligan said.

A whistleblower at the depot, meanwhile, described multiple safety hazards to The Sun newspaper Wednesday, and called the facility a “ticking bomb.”

But an official investigating the fire said he had had no “grave concerns” about conditions at the depot before the blaze, and the county’s fire chief called the union’s claims “utter nonsense.”