Sources close to the investigation tell Forbes that Transocean engineers had last checked the blowout preventer 10 days before the rig explosion, and that BP believes the device suffered a leak of hydraulic pressure, preventing it from being able to shut the well.

FORBES: Fingers Crossed: Will BP's Box Stop Oil Spill?

With that leak sealed, the plan is to inject heavyweight "kill mud" into the lower part of the blowout preventer, followed by cement that will effectively entomb the well.

WSJ: Spill Fight Shows Progress