The report's findings could hurt the company's effort to fend off criminal charges and billions of dollars in penalties.
Eleven rig workers were killed in the April 2010 explosion, and some 200 million gallons of crude spewed from the bottom of the sea.
In the report, the primary cause of the disaster was identified as the failure of the cement seal in the well. While it was Halliburton's job to mix and test the cement, BP had the final word.
The report says the oil company made several decisions that complicated the cementing operations and added risk. Also, the company didn't communicate those decisions to the owner of the rig, Transocean.
The findings do, however, show other companies were at fault for the spill, including Transocean.
Transocean was accused of being deficient in preventing or limiting the disaster, in part by bypassing alarms and automatic shutdown systems.
Halliburton, the contractor responsible for mixing and testing the cement, was faulted as well.
BP responded to the report by saying it is time for "other parties to acknowledge their roles in the accident and make changes to help prevent similar accidents in the future." Transocean said it takes exception to any criticism of its drill crew. Halliburton did not comment.
The findings will be used to shape reforms in offshore drilling safety and regulation. They will also be used by lawyers for victims involved in court battles over the oil spill, and by government agencies considering charges and penalties.
The Associated Press contributed to this report.