It’s too early to know for certain if it was an accident or if it was terrorism.  And even if it was determined to be terrorism there is a good chance that knowledge would be restricted.  In any case, it sure was a big explosion (latest news here):

A series of explosions rocked a major oil depot just north of London just before dawn Sunday, injuring 36 people.

Huge balls of fire shot into the sky and the area was filled with clouds of thick, black smoke.

People living nearby have been forced to leave their homes in Hemel Hempstead.

Some residents said they heard a small plane flying overhead just before the first blast. But Hertfordshire police said rumours that the aircraft was involved were unfounded.

Witnesses said there were three explosions at the Buncefield fuel depot. The blasts were heard as far away as the Netherlands.

Some firefighters said that this was the biggest fire they’ve ever tackled.

Thick clouds of smoke were spreading to the southeast and southwest, but were not believed to be toxic.

The oil depot supplies fuel for a large part of southeast England. However, a spokesperwosn for oil giant BP said there would be no problems with fuel shortages.

And if it does happen that it was a terrorist attack the VPC and Brady Bunch should take note that this sort of thing happens even where .50 BMG’s are banned.  It’s the people not the guns.