A "complex and interlinked" series of events involving mechanical failures and human judgments led to the Gulf of Mexico oil disaster, a report by oil giant BP said today.
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 placed some blame on rig owner Transocean and cement contractor Halliburton.
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 today'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 has 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.
It is not the first time that blame has been shared, with BP, Transocean and Halliburton all pointing the finger at each other at a US Congress meeting in May.
But this report is not the final word on the causes of the explosion, as several divisions of the US government, including the Justice Department, Coast Guard and Bureau of Ocean Energy Management, Regulation and Enforcement are also investigating.
In addition, the failed blow-out preventer highlighted in today's findings was only raised from the water on Saturday, and awaits further analysis.
BP has already spent 8 billion US dollars (£5.2 billion) trying to contain the disaster, and has forecast that it will eventually cost the group more than 32 billion US dollars (£20.9 billion), after clean-up costs and compensation are taken into account.
The crisis cost former chief executive Mr Hayward his job after a series of PR blunders and he will make way in October for fellow board member Bob Dudley, who becomes BP's first overseas boss.
Following publication of today's findings, Mr Dudley said: "We deeply regret this event. We have sought throughout to step up to our responsibilities.
"We are determined to learn the lessons for the future and we will be undertaking a broad-scale review to further improve the safety of our operations."
Before the publication of today's report, ratings agency Fitch upgraded BP's debt from its lowest investment grade, BBB, to A.
Fitch said the upgrade "primarily reflects an end to the threat of further leaks from the Macondo well in the Gulf of Mexico".Reuse content