Backlash against BP spill report
BP's investigation into the Gulf of Mexico spill sparked a backlash from its contractors and US politicians who accused the oil giant of attempting to shift some of the blame.
The internal inquiry, led by the company's head of safety and operations, Mark Bly, found BP was responsible in part for the tragedy, but also pointed the finger at rig owner Transocean and cement contractor Halliburton.
The report said a "complex and interlinked" series of events involving mechanical failures and human judgments led to the disaster.
The explosion on April 20 killed 11 workers and caused an estimated 4.9 million barrels of oil to gush into the Gulf - the largest offshore spill in history.
Commenting on yesterday's findings, outgoing chief executive Tony Hayward, who was forced to stand down in the wake of the disaster, said: "The investigation report provides critical new information on the causes of this terrible accident. It is evident that a series of complex events, rather than a single mistake or failure, led to the tragedy. Multiple parties, including BP, Halliburton and Transocean, were involved."
The four-month investigation found shoddy cement work at the bottom of the Deepwater Horizon well failed to hold gas and oil in its reservoir, which leaked into the casing.
BP and Transocean employees then incorrectly accepted negative pressure readings in the crucial minutes before the explosion - meaning they did not spot the gas leak, the inquiry found.
Further mechanical failures then allowed gas to be vented directly on to the rig rather than being diverted overboard - where it ignited. The rig's blow-out preventer, a protective valve, should have sealed the well but failed to operate.
Based on its key findings, the investigation team proposed a total of 25 recommendations designed to prevent such an incident. The recommendations are directed at strengthening blow-out preventers, well control, pressure-testing for wells, emergency systems, cement testing, rig audit and verification, and personnel competence.
Accepting full responsibility for the disaster could lead to BP being found guilty of gross negligence and fined up to 20 billion US dollars (£13 billion) so it was always likely to maintain its stance that other parties were involved.