A US federal report has blamed the worst oil spill in US history on "key mis-steps", poor leadership and a poor cement job by BP and its contractors.
BP was "ultimately responsible" for rig safety, with Transocean responsible for safe operations and worker safety.
BP tried to save time and money at the cost of safety, and Transocean operated normally despite the hazards, it said.
"Multiple causes, involving multiple parties, including Transocean and Halliburton" were to blame, BP said.
The April 2010 Deepwater Horizon blast killed 11 workers and spilled four million barrels of oil into the Gulf of Mexico.
It took three months and highly complex undersea engineering effort to plug the well and stop the oil flow.
The report, issued by the the Bureau of Ocean Energy Management, Regulation and Enforcement and the US Coast Guard Joint Investigation Team, echoes previous reports in concluding that multiple factors contributed to the disaster.
The explosion was "the result of poor risk management, last-minute changes to plans, failure to observe and respond to critical indicators, inadequate well control response, insufficient emergency bridge response training by companies and individuals responsible," it said.
As well as labelling BP "ultimately responsible" for safety, the report blames Halliburton for conducting a poor cement job to seal the well that failed in the run-up to the blast.
Commissioned in April 2010, just days after the explosion at the rig, the report also offers recommendations to improve the way the offshore drilling industry is regulated.
A number of other reports have been published in the wake of the oil spill.
Correspondents say the latest report may also offer a hint at the direction of any legal ramifications for BP and others, and could lead to more charges against them by the US government.
In December 2010, the US Department of Justice sued BP and eight other companies for billions of dollars in damages from the spill. The lawsuit asked that the companies be held "liable without limitation".
The report is also likely to be combed over by corporate legal teams working on lawsuits the Deepwater Horizon companies have brought against each other.
In April 2011, a year after the blow-out at the Macondo well, BP sued Transocean for $40bn (£25.3bn) in damages - the start of a long legal battle to spread the costs of the clean-up that, until then, had been covered by BP alone.
Transocean spokesman Brian Kennedy told the BBC that while the company agreed with some of the report's conclusions, they took "strong exception to criticisms of the Horizon drill crew, nine of whom perished fighting to save their fellow crewmembers and the rig, for the actions they took in the face of such an unprecedented emergency".
Halliburton and BP have also filed several lawsuits against each other - over the issue of the faulty cement seal, defamation and the concealment of vital information that could have prevented the disaster.