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

In a 193-page report posted on its website on 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.

BP spread the blame around, and even was critical of its own workers’ conduct, but it defended the design of its well and it was careful in its assessments. It already faces hundreds of lawsuits and billions of dollars in liabilities. In public hearings, it had already tried to shift some of the blame to rig owner Transocean Ltd. and cement contractor Halliburton. BP was leasing the rig from Transocean and owned the well that blew out.

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.

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 were not be part of BP’s report.

The April 20 rig explosion killed 11 workers and led to 206 million gallons (nearly 800 million liters) 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.