How did the oil rig explode in the gulf of Mexico?

An uncontrolled release of hydrocarbons ignited on the drill floor. Primary well control was provided by the completion fluid, but this was being changed out during normal testing operations. Primary well control was then the cement job on the liner which was woefully inadequate in design (BP US) and execution (Halliburton). With no primary well control, then the rig relied on secondary well control, the blowout preventer (Transocean). This also failed. Due to the explosion, damage and loss of the Deepwater Horizon there was no way to recover control. Therefore, tertiary well control was implemented; drilling a relief well into the toe of the uncontrolled well and killing it from the bottom.