The 193-page report was posted on the company's website even though investigators have not yet begun to fully analyze a key piece of equipment, the blowout preventer, that should have cut off the flow of oil from the ruptured well but did not.
That means BP's report is far from the definitive ruling on the blowout's causes, but it may provide some hint of the company's legal strategy - spreading the blame around between itself, rig owner Transocean, and cement contractor Halliburton - as it faces hundreds of lawsuits and possible criminal charges over the spill. Government investigators and congressional panels are looking into the cause as well.
Members of Congress, industry experts and workers who survived the rig explosion have accused BP's engineers of cutting corners to save time and money on a project that was 43 days and more than $20 million behind schedule at the time of the blast.
BP's report acknowledged, as investigators have previously suggested, that its engineers and employees of Transocean misinterpreted a pressure test of the well's integrity. It also blamed employees on the rig from both companies for failing to respond to warning signs that the well was in danger of blowing out.
Outgoing BP chief Tony Hayward, who is being replaced Oct. 1 by American Bob Dudley, said in a statement that there was a bad cement job and a failure of a barrier at the bottom of the well that let oil and gas leak out.
Transocean blasted BP's report, calling it a self-serving attempt to conceal the real cause of the explosion, which it blamed on what it called "BP's fatally flawed well design."
"In both its design and construction, BP made a series of cost-saving decisions that increased risk - in some cases, severely," Transocean said.
Transocean said its own investigation will be concluded when all of the evidence is in, including critical information the company has requested of BP but has yet to receive.
New Orleans attorney Scott Bickford, who represents relatives of a worker who died in the explosion and a worker who survived the blast, said he found no surprises in the report.
"My knee-jerk reaction is that there was no huge smoking gun they found that hasn't already been discussed," he said.
BP shares were up 2 percent at 414.95 pence ($6.41) in London shortly after the report was made public Wednesday.
Several divisions of the U.S. government, including the Justice Department, Coast Guard and Bureau of Ocean Energy Management, Regulation and Enforcement, are also investigating the explosion.
The blowout preventer was raised from the water off the coast of Louisiana on Saturday. As of Tuesday afternoon, it had not reached a NASA facility in New Orleans where government investigators planned to analyze it, so those conclusions were not part of BP's report.
The rig explosion killed 11 workers and sent 206 million gallons of oil spewing from BP's undersea well.
Investigators know the explosion was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before igniting.
But they don't know exactly how or why the gas escaped. And they don't know why the blowout preventer didn't seal the well pipe at the sea bottom after the eruption, as it was supposed to.
The details of BP's internal report were closely guarded - and only a short list of people saw it ahead of its release.
There were signs of problems prior to the explosion, including an unexpected loss of fluid from a pipe known as a riser five hours before the explosion that could have indicated a leak in the blowout preventer.
Witness statements show that rig workers talked just minutes before the blowout about pressure problems in the well.
At first, nobody seemed too worried, workers have said. Then panic set in.
Workers called their bosses to report that the well was "coming in" and that they were "getting mud back." The drilling supervisor, Jason Anderson, tried to shut down the well.
It didn't work. At least two explosions turned the rig into an inferno.
In its report, BP defended the well's design, which has been criticized by industry experts.
Other findings in the BP report include:
-Flammable fluids rising up the pipe toward the Deepwater Horizon rig were directed to a system that allowed gas to vent onto the rig, and that gas was then circulated by the air conditioning, heating and ventilation systems. BP says that if the crew had directed the fluids overboard, there might have been more time to respond to the pending disaster and the consequences of the accident may have been reduced.
-BP concluded that a "more thorough review and testing by Halliburton" and "stronger quality assurance" by BP's well team well might have identified potential flaws and weaknesses in the design for the cement job.
-BP counters the concerns that were raised prior to the explosion by Halliburton over the potential for a severe gas flow problem if a BP plan was used. Halliburton and BP were at odds over a key device, known as a centralizer, that is used as part of the process to plug a deepwater well like the oil giant was doing at the time of the disaster. Halliburton's well design expert testified previously he told BP officials April 15 - five days before the well blew - that fewer centralizers would cause a bigger gas flow problem. BP rejected Halliburton's recommendation to use 21 centralizers. Instead, BP used six. In its report Wednesday, BP said the decision likely did not contribute to the cement's failure.
In June, the House Committee on Energy and Commerce's chairmen said it was BP that made five crucial decisions before the Deepwater Horizon well blowout that "posed a trade-off between cost and well safety." One of those decisions: BP opted against conducting a certain kind of test of the integrity of a cement job at the well. The test would have cost more than $128,000 and taken 9 to 12 hours to perform, the committee's letter notes.
In May, senior BP drilling engineer Mark Hafle told the Coast Guard and Bureau of Ocean Energy Management investigators that BP didn't order the test even though more than 3,000 barrels of mud had been lost while drilling, a possible warning sign.
The committee also criticized BP's well design.
