British oil giant BP on Wednesday released a long-awaited report on an internal investigation into the causes of the Deepwater Horizon explosion, blaming multiple factors by BP and other companies involved for the disaster that killed 11 workers and unleashed the worst oil spill in U.S. history.

Decisions made by "multiple companies and work teams" contributed to the accident which arose from "a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces, " the company said in the report based on a four-month investigation.

The report cited the failure of the type of cement slurry that was used, along with a device know as a shoe track barrier, saying they had "failed to contain hydrocarbons within the reservoir, as they were designed to do, and allowed gas and liquids to flow up the production casing."

The report also said that BP and Transocean workers " incorrectly accepted" the results of the negative pressure test. " Over a 40-minute period, the Transocean rig crew failed to recognize and act on the influx of hydrocarbons into the well until the hydrocarbons were in the riser and rapidly flowing to the surface," it said.

Meanwhile, the report said after the well-flow reached the rig it was routed to a mud-gas separator, causing gas to be vented directly on to the rig rather than being diverted overboard, creating "a potential for ignition which the rig's fire and gas system did not prevent."

The report also confirmed that the rig's blow-out prevent on the seabed failed to operate in the accident "probably because critical components were not working."

"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," said BP' s outgoing CEO Tony Hayward, who was heavily criticized for his handling of the crisis.

BP-leased Deepwater Horizon drilling rig exploded off the Louisiana coast in late April, killing 11 workers and unleashing the worst spill in U.S. history, with millions of gallons of oil gushing into the Gulf of Mexico from the ruptured underwater well.