"(A) sequence of failures involving a number of different parties led to the explosion and fire which killed 11 people and caused widespread pollution in the Gulf of Mexico earlier this year," BP said in a release posted on its Web site.
The internal report concluded decisions made by "multiple companies and work teams" led to the failure that resulted in millions of barrels of oil leaking into the gulf, fouling the environment and damaging the gulf economy.
The April 20 accident resulted from "a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces," the report said.
During congressional hearings into the disaster, the three companies involved -- BP, Halliburton and Transocean -- pointed fingers at each other.
The report resulted from a four-month investigation lead by Mark Bly, BP's safety and operations head.
Among its findings:
-- Cement and shoe track barriers, particularly the cement used, at the bottom of the Macondo well failed to contain hydrocarbons within the reservoir, allowing gas and liquids to flow up the production casing.
-- Results of the negative pressure test were incorrectly accepted by BP and Transocean, even though well integrity had not been established.
-- During a 40-minute period, the Transocean rig crew didn't recognize and act on the influx of hydrocarbons into the well until the hydrocarbons were in the riser and rising to the surface.
-- After the well flow reached the rig, it was routed to a mud-gas separator, causing gas to be vented onto the rig instead of overboard.
-- The flow of gas into the engine rooms through the ventilation system created a potential for ignition that the rig's fire and gas system did not prevent.
-- After an explosion and fire disabled its crew-operated controls, the rig's blow-out preventer on the seabed should have activated to seal the well, but failed to operate, likely because components weren't working.
"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," outgoing BP Chief Executive Officer Tony Hayward said.
Incoming BP CEO Bob Dudley said, "We have accepted all the recommendations and are examining how best to implement them across our drilling operations worldwide."
Dudley said BP will invest "whatever it takes" to improve the safety of its operations.