The rig’s blowout preventer failed to prevent a massive oil leak
BP is to release an internal investigation into the oil spill in the Gulf of Mexico, the worst ecological catastrophe in recent US history.
The investigation has taken five months, and is expected to blame BP as well as other companies involved in the drilling operation.
Safety equipment on the Deepwater Horizon rig failed after an explosion on 20 April, in which 11 people died.
The report will play a key role in how BP defends itself in legal proceedings.
An estimated 4.9m barrels of oil leaked into the waters of the Gulf after the blast, with only 800,000 barrels being captured.
A cap was used to seal the top of the wellhead on 15 July, and an operation to permanently seal the ruptured oil well is due to take place in the next few weeks.
BP says dealing with the aftermath of the spill has cost $8bn (£5.2bn), and it has already paid out about $399m in claims to people affected by the spill.
The US Department of Justice is conducting a criminal investigation.
The internal BP investigation has been prepared by the company’s head of safety, Mark Bly.
It is expected his report will focus on a complex combination of breakdowns and mistakes rather than just one factor, says the BBC’s Andy Gallacher in Miami.
That could include equipment that might have failed, or engineers who missed warning signs.
BP hired Transocean and Halliburton to work on the well, and it is thought they could come in for criticism, too, our correspondent says.
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