Oil giant BP isn't the only company to blame for the massive oil spill that has polluted waters off the Gulf of Mexico, according to an investigation by the organization.

Instead "multiple companies" are to blame for a sequence of failures that involve failed human judgement, engineering design, mechanical failures and operational implementation, the 193-paged report says.

The report was commissioned after a sea floor oil rig exploded on April 20, 2010, causing the largest accidental spill in history. About 4.9 million barrels of crude oil were released into the gulf before it was capped on July 15.

Eleven platform workers were killed in the explosion and 17 others were injured.

The report, which was made public on Wednesday, noted a chain reaction of failed incidents that led to the explosion and continued after the fire had been set.

"Even after explosion and fire had disabled its crew-operated controls, the rig's blow-out preventer on the sea-bed should have activated automatically to seal the well," the report says. "But it failed to operate, probably because critical components were not working."

BP's outgoing CEO Tony Hayward explained in an internal memo that the design of the Maconda well was not flawed.

"To put it simply, there was a bad cement job and a failure of the shoe track barrier at the bottom of the well, which let hydrocarbons from the reservoir into the production casing. The negative pressure test was accepted when it should not have been, there were failures in well control procedures and in the blow-out preventer; and the rig's fire and gas system did not prevent ignition.

"Based on the report, it would appear unlikely that the well design contributed to the incident, as the investigation found that the hydrocarbons flowed up the production casing through the bottom of the well," he continued.

BP's findings sparked some criticism from U.S. officials.

Congressman Edward Markey (D-Mass.) issued a statement Wednesday from Ottawa where he is visiting, saying he was skeptical about BP-funded investigation.

"This report is not BP's mea culpa," he said. "Of their own eight key findings, they only explicitly take responsibility for half of one. BP is happy to slice up the blame, as long as they get the smallest piece."

Markey said BP is right to say that the blame should be shared but that the problems started decades ago and stretch across the entire oil industry.

He blamed the oil industry for giving assurances that this massive of an oil spill wouldn't happen and could be contained if it did. He also noted the "overly-cozy relationships established between the oil industry and regulators."

Markey said the truth will come out with further investigation.

"I look forward to seeing the final results of the multiple other investigations not funded by BP or the other companies involved in this disaster," he said. "Those are the reports that will tell the real story of this disaster, and give us the lessons we need to create laws that will prevent this type of accident from happening again."

BP's report did include 25 recommendations that would help prevent a similar occurrence in the future. The recommendations suggest cement testing, rig audits and strengthening assurance on blow-out preventers, well control and pressure testing. There is also a recommendation to review personnel competence.

The BP investigation is one of multiple ongoing probes into the disaster. Departments within the U.S. government -- including the justice department, Coast Guard and Bureau of Ocean Energy Management, Regulation and Enforcement -- are conducting their own investigations.

BP's study took four months to complete. It was led by Mark Bly, the company's head of safety operations but included a team of more than 50 specialists drawn from inside and outside the BP organization.

The report noted that "additional relevant information may be forthcoming, for example, when Halliburton's samples of the cement used in the well are released for testing and when the rig's blow-out preventer is fully examined now that it has been recovered from the sea-bed."