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A broad and comprehensive federal report into the deadly Deepwater Horizon oil rig explosion, and the subsequent leak of millions of gallons of crude oil, puts the majority of the blame on BP's shoulders.

The report was released Wednesday by the U.S. Bureau of Ocean Energy Management, Regulation and Enforcement. The investigation was carried out over a year by a team made up of the coast guard and federal offshore drilling regulators.

The report is clear it was BP's role to ensure the safety of the workers, and the environment.

"BP, as the designated operator under BOEMRE regulations, was ultimately responsible for conducting operations at Macondo in a way that ensured the safety and protection of personnel, equipment, natural resources, and the environment," the report states.

The blame for the April 20, 2010 blast -- which caused 11 deaths and the spill of 757 million litres of crude oil -- is spread around to a certain degree, however.

The report states that Transocean, which owned the rig, was specifically tasked with conducting safe operations and protecting personnel. Halliburton, on contract to BP, was responsible for the cement sealing job, which ultimately failed, as well as monitoring. And a third company, Cameron International, was responsible for the design of the rig's blowout preventer stack, which didn't work properly.

The blowout that caused the explosion and sparked a massive blaze on the rig was primarily caused by the failure of the cement barrier on the ocean floor, which was put in place a day before the disaster, investigators found.

"The failure of the cement barrier allowed hydrocarbons to flow up the wellbore, through the riser and onto the rig, resulting in the blowout," the report states, while acknowledging it isn't clear why the cement failed.

That sequence of events was complicated by poor risk management, last-minute changes to plans, failure to observe and respond to critical indicators, inadequate well control response and insufficient emergency training, the report adds.

In the days leading up to the April 20 explosion the following decisions were made that may have led to the disaster, the report says.

• The use of only one cement barrier: BP had no backup plan for sealing the well in the event that the barrier failed, which it ultimately did.

   

• The location of production casing: The steel tube known as production casing, used to protect the integrity of an oil well, was placed in an area that created "additional risk of hydrocarbon influx" -- which eventually caused the blowout.

   

• The decision to install a lock-down sleeve: BP was preparing to temporarily abandon the well, which required the installation of a lock-down sleeve, which actually increased the risks due to mud displacement and other factors.

   

• The production casing cement job: In perhaps its bluntest finding, the report found the cement was not installed "in accordance with industry-accepted recommendations."

The risks associated with all the above decisions were also not properly communicated by BP to Transocean, and the crew on the oil rig simply was not aware of the danger they were in.

The report says those in the most danger -- the employees working on the drilling floor at the centre of the rig -- weren't privy to warning signs being detected by engineers and had no idea how dire their situation was.

Investigators also found that BP's decisions were often made without regard for risk, and often didn't follow the internal processes used to evaluate danger.

The panel recommended a number of changes to improve safety. Among them, the report recommends have at least two barriers be required -- a cement one, as was in place at Deepwater Horizon, but also a mechanical barrier, which the Deepwater Horizon did not have in place.