Millions of gallons of oil spilled into the Gulf of Mexico after the explosion on the drilling platform [GALLO/GETTY]
No single factor caused the Gulf of Mexico oil spill and multiple companies and individual work teams contributed to the "accident", BP has said in a report published after an internal investigation.
The energy giant stopped short of admitting gross negligence, but proposed 25 recommendations designed to prevent a repetition of the explosion at the Deepwater Horizon rig that led to the spill of 4.9 million barrels of oil into the sea.
The April 20 explosion left 11 people dead and efforts to stop the massive oil spill took several months to succeeded.
BP said in the report published on Wednesday that the accident rose from "a complex and interlinked series of mechanical failures, human judgements [and] engineering design".
The four-month probe, led by Mark Bly, BP's head of safety and operations, is viewed as key to how the company will defend itself against legal proceedings involving the spill.
"This report likely does its job in providing ammo [ammunition] for BP in future court cases, where the avoidance of the charge of 'gross negligence' is critical," said Peter Hutton, an oil market analyst at NCB Stockbrokers.
'Bad cement job'
In the report, BP also blamed the rig's owner Transocean and Halliburton, which had cemented the well.
"The investigation report provides critical new information on the causes of this terrible accident," Tony Hayward, BP's outgoing chief executive, said.
"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."
Hayward said key failings included a "bad cement job" at the bottom of the well that allowed gas and liquids to flow up the production casing.
The Gulf Restoration Network, an environmental organisation that seeks to protect the natural resources of the Gulf region, rejected the outcome of the internal investigation.
"Instead of blaming their own decision not to use more sensors on the well, they blamed the concrete," Aaron Viles, campaign director for the Gulf Restoration Network, told Al Jazeera.
"They are positioning themselves to have another tool as they seek to share responsibility, and ultimately liability, with Transocean and Halliburton," he said, adding that "it is pretty transparent, similar to my three year old daughter when she tells me it is not time for bed yet.
"When they said it was a succession of mistakes that led to this blow-out, … I think that is true. But I do think that they put themselves in a pretty good light as you work through this report."
BP denied it was shifting blame and accepted that one of its representatives, in conjunction with Transocean, had incorrectly interpreted a safety test that should have flagged up risks of a blowout.
The company said last week it had spent $8bn so far responding to the spill, but analysts expect the final bill to run to tens of billions of dollars.
Manouchehr Takin, a specialist in petroleum production with the Centre for Global Energy Studies, said BP's internal report was "impartial as a technical evaluation".
"One cannot blame one or two, and one cannot say there is no one to blame," he said.
"BP being the licence holder are definitely responsible for it but there are some issues which if we go through later on could be partly apportioned to other companies."
The leaking Macondo well has now been secured but the disaster is being examined in a string of court cases and probes, including a criminal investigation being carried out by the US department of justice.
US politicians have accused the oil giant of sacrificing safety to improve its profit margin - a charge Hayward denied during a session in congress in June.
The US government is currently conducting its own investigation into the oil disaster.
Source: Al Jazeera and agencies