The presidential commission investigating the Deepwater Horizon oil spill says BP and its cement contractor, Halliburton knew the cement mixture used in the construction of the well could allow oil leaks.

According to a letter from Fred Bartlit, Jr., the commission’s lead investigator, Halliburton carried out four tests of the cement mixture between February and April of this year, shortly before the rig exploded, and three of them showed the mixture to be faulty. A fourth, final one showed the mixture to work, but BP apparently only ever saw the results of one of the tests — one that failed.

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CNN reports:

According to the letter, the cement was poured to stabilize the well on April 19 and 20, the day of the explosion on the Deepwater Horizon rig above that killed 11 workers and caused the worst oil spill in U.S. history. “Halliburton and BP both had results in March showing that a very similar foam slurry design to the one actually pumped at the Macondo well would be unstable, but neither acted upon that data,” the letter said. “Halliburton (and perhaps BP) should have considered redesigning the foam slurry before pumping it at the Macondo well,” the letter continued.

The news “appears to conflict with statements made by Halliburton Co., which has said its tests showed the cement mix was stable. The company instead has said BP’s well design and operations are responsible for the disaster,” reports AP.

The letter places primary blame for the Deepwater Horizon explosion on the faulty cement well, reports MSNBC.

That improper cement work “may have contributed to the blowout” on April 20 that killed 11 workers and led to the largest offshore oil spill in history, the staff stated in the first official finding of responsibility for the disaster. Other factors have also been cited as possibly contributing to the explosion, among them a faulty blowout preventer and BP’s decisions to use fewer stability rings on the well piping and seawater instead of heavier mud to plug the well until it was to be used for production. The staff noted, however, that had the cement done its job it would “have prevented hydrocarbons from entering the well” and triggering the explosion.

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The presidential commission into the oil spill, which is headed up by former Florida Sen. Bob Graham and ex-EPA head William Reilly, is expected to report on its initial findings in early November.