Up until this point, it's been common knowledge that an explosion on the Deepwater Horizon led to the oil leak in the Gulf of Mexico. But now, BP has released a report that lists the causes which led to the explosion, the death of 11 people, and the worst oil leak in U.S. history. 

"The investigation report provides critical new information on the causes of this terrible accident," said Tony Hayward, BP's chief executive. "It is evident that a series of complex events, rather than a single mistake or failure, led to the tragedy. Multiple parties, including BP, Halliburton and Transocean, were involved."

Mark Bly, BP's Head of Safety and Operations, along with over 50 technical and other specialists that are both internal and external to BP, led a four-month investigation in order to compile information for the report and list the multiple causes associated with BP's oil spill.

According to the report, several factors led to the oil leak, such as cement and shoe track barriers failing to contain hydrocarbons within the reservoir, allowing liquids and gas to to flow up production casing. The Transocean crew did not act on the influx of hydrocarbons into the well until they were in the riser; well integrity had not yet been established, and BP and Transocean accepted the results of the negative pressure test incorrectly; the rig's fire and gas system failed to detect and prevent ignition when the flow of gas entered engine rooms through the ventilation systems; and the rig's blow-out preventer did not activate automatically to seal the well after the explosion disabled crew-operated controls. 

The report concluded that several different work teams and companies made poor decisions which led to "a complex and interlinked series of mechanical failures, human judgements, engineering design, operational implementation and team interfaces."

The report not only listed causes of the disaster, but also provided recommendations for the companies involved (specifically BP) to ensure that proper precautions are taken to prevent this type of event from ever occurring again. The recommendations aimed to improve well control, emergency systems, blow-out preventers, cement testing, personnel competance, pressure-testing for well integrity and rig audit and verification. 

"I believe this report will be of significant value in helping the overall understanding of how this tragedy occurred," said Carl-Henric Svanberg, Chairman of the Board. "It is if the utmost importance to the Board to ensure that BP learns from this and further enhances the safety of its operations for the future."