"mechanical failure" does NOT equal "accident"
An accident implies random chance and that the cause is inexplicable ("anything that happens suddenly or by chance without an apparent cause" per Princeton WordNet). Even BP, which would certainly love to be able to blame it all on God, tells us that they expect an investigation to reveal the cause.
But hey, we're engineers here, and we can go farther. The failure will be found to have one or both of two possible roots: Design errors; or errors in fabrication/construction or materials (e.g. invisible cracks in metal). Both are, at root, human failings; and both are preventable.
One thing for sure, this was no accident.