Fire boat crews battle the blazing offshore oil rig Deepwater Horizon off the coast of Louisiana on April 21. (U.S. Coast Guard/Reuters)

BP says a "sequence of failures" involving "multiple companies" led to the explosion and fire that killed 11 people and caused a massive Gulf of Mexico oil spill.

In a 193-page report released Wednesday and posted on its website, the company said the accident arose from “a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces.”

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

The April 20 explosion and sinking of the Deepwater Horizon rig led to the leak of millions of barrels of oil into the Gulf became the worst ecological disaster in the United States in recent memory.

BP's report will likely lay the foundation for its legal defence in future lawsuits.

BP has already used public hearings to try to shift some of the blame to rig owner Transocean Ltd. and cement contractor Halliburton.

In its report, BP says factors it believes led to the catastrophe include:

• Cement and equipment at the bottom of the well failed, allowing the upward flow of hydrocarbons.
• Pressure tests were incorrectly accepted by Transocean rig crew and BP staff.
• The Transocean crew failed to realize that hydrocarbons were flowing upward.
• The fluids were not redirected overboard, but resulted in gas venting onto the rig.
• The rig's own ventilation system probably pumped gas to the engine room, a possible ignition source.
• Three methods of closing the well's blowout preventer failed, with BP citing problems with testing and maintenance of the equipment.

BP spends billions after spill

The blowout preventer, which was meant to stop the flow of oil and gas to the surface but failed to do so after the explosion, was lifted from the water on Sept. 4. The preventer is on its way to a NASA facility in New Orleans for analysis by government investigators.

Transocean responded to the report by saying the fault lay with "BP's fatally flawed well design."

'BP blaming others for the Gulf oil disaster is like Bernie Madoff blaming his accountant.' —Robert Gordon

"In both its design and construction, BP made a series of cost-saving decisions that increased risk — in some cases, severely," Transocean said.

Transocean is performing its own investigation, and said it is still waiting on information it has requested from BP.

A lawyer who represents more than 1,000 clients with claims against BP, including fishermen, hotels and restaurants, slammed BP after the report was released.

"BP blaming others for the Gulf oil disaster is like Bernie Madoff blaming his accountant," Robert Gordon said.

BP has also been adamant it is doing its part to manage the environmental catastrophe.

The company said it has shelled out $8 billion US to help handle the aftermath of the spill and paid hundreds of millions of dollars in compensation claims to people along the Gulf coast whose livelihoods have been affected.

The BP report will not be the final look into the events surrounding the leak.

Several divisions of the U.S. government, including the Justice Department, coast guard and the Bureau of Ocean Energy Management, Regulation and Enforcement, are also investigating.

with files from The Associated Press