Oil giant BP PLC says in an internal report that multiple companies and work teams contributed to the massive Gulf of Mexico spill that fouled waters and shorelines for months.

In its 193-page report posted on its website Wednesday, the British company describes the incident as an accident that arose from a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces.

The report was generated by a BP team led by Mark Bly, BP's head of safety and operations.

BP's report is far from the final word on possible causes of the explosion, as 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.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

NEW ORLEANS (AP) _ Oil giant BP PLC on Wednesday planned to release the conclusions of its internal investigation into the rig explosion that killed 11 workers and led to the massive Gulf of Mexico spill.

BP was striking first with a detailed report about what it believes went wrong on the Deepwater Horizon on April 20. It comes just days after a key piece of evidence in the explosion was raised to the surface but has not yet been analyzed. It also comes as the blame game heats up in Washington and in the courtroom.

BP was unlikely to place too much onus on itself, given the hundreds of lawsuits and billions of dollars of liabilities it faces. In public hearings, the company has tried to shift some blame to rig owner Transocean Ltd. and cement contractor Halliburton. BP was leasing the rig from Transocean and owned the well that blew out a mile under the Gulf surface, spewing some 206 million gallons of oil into the water over three months.

BP's report is far from the final word on possible causes of the explosion, as 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.

Also, a key piece of the puzzle _ the blowout preventer that failed to stop the oil from leaking from the well off the Louisiana coast _ was raised from the water 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 will not be part of BP's report.

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 have been closely guarded _ and only a short list of people saw it ahead of its release. Mark Bly, BP's group head of safety and operations, compiled the report and said in May that there was a gas plume before the explosion.

"This was a pretty large plume of gas ... I think it was a large enough where there was a high likelihood it would have ignited," he said.

There were other signs of problems, 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.

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.