Washington (CNN) -- Oil giant BP and contractor Halliburton knew of potential flaws in the cement slurry used to reinforce the oil well below the Deepwater Horizon rig before it exploded in April, according to a letter Thursday from the lead investigator for a federal probe of the Gulf oil disaster.

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The letter from Fred Bartlit Jr. to the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling said that tests in February on a cement slurry similar to what was used on the Macondo well showed instability -- and that both companies had the data.

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