BP concluded its investigation of the April 20 explosion that caused an oil spill in the Gulf, citing faulty cementing, a misread pressure test and an improperly maintained blowout preventer.

BP stated that its team aboard the Deepwater Horizon oil rig “incorrectly accepted” results of a negative pressure test aboard the rig before the blast, but the company’s internal report assigns a lot of blame on to the rig owner Transocean and cementing contractor Halliburton. BP, Transocean and Halliburton have repeatedly pointed fingers at each other since the explosion, which killed 11 workers and resulted in an estimated 4.9 million barrels of oil spilling into the Gulf.

“The team did not identify any single action or inaction that caused this accident,” BP’s report states. “Rather, a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces came together to allow the initiation and escalation of the accident. Multiple companies, work teams and circumstances were involved over time.”

BP acknowledged that its team leaders aboard the rig should not have accepted the results of a negative pressure test on the cement seal.

“In retrospect, pressure readings and volume bled at the time of the negative-pressure test were indicators of the flow-path communication with the reservoir, signifying that the integrity of these barriers had not been achieved,” the report states.

BP’s investigation found that Halliburton used a “likely unstable” cement mix that was not fully tested before it was used. BP’S head of safety and operations, Mark Bly said that Halliburton “did not conduct comprehensive lab tests that could’ve identified potential problems with the cement.”

“We believe that BP and Halliburton working together should have better identified and addressed the issues underlying the cement job.”

Bob Dudley, who is replacing BP CEO Tony Hayward, said Wednesday’s report “makes that conclusion even clearer…. We deeply regret this event. We have sought throughout to step up to our responsibilities. We are determined to learn the lessons for the future and we will be undertaking a broad-scale review to further improve the safety of our operations.”