BP deflected much of the blame for a rig blast that led to the United States' worst-ever oil spill, releasing an internal report on Wednesday which said that drilling contractor Transocean had missed danger signs.
BP defended its much-criticized well design and said failures on the rig, operated by Transocean, led to gas swamping the platform and creating the conditions for the explosion.
"Over a 40-minute period, the Transocean rig crew failed to recognize and act on the influx of hydrocarbons into the well," BP said in a statement.
BP also criticized the cementing of the well, conducted by Halliburton, and repeated previous criticism of the blowout preventer -- a key piece of equipment operated by Transocean.
BP accepted that its representatives, in conjunction with Transocean, had incorrectly interpreted a safety test which should have flagged up risks of a blowout.
"To put it simply, there was a bad cement job," Chief Executive Tony Hayward said in a statement.
"It would appear unlikely that the well design contributed to the incident, as the investigation found that the hydrocarbons flowed up the production casing through the bottom of the well," Hayward added.
The Deepwater Horizon drilling rig exploded on April 20, killing 11 workers. It sunk two days later, unleashing a surge of crude that lasted until the well was capped on July 15, after 4.9 million barrels of oil had leaked into the sea.
BP said last week it had spent $8 billion so far responding to the spill but analysts expect the final bill to run to tens of billions.