According to a letter Thursday from Fred Bartlit, Jr., the lead investigator for a federal probe of the Gulf oil disaster, BP and Halliburton knew of potential flaws in the cement slurry used to reinforce the oil well below the Deepwater Horizon rig before it exploded on April 20, 2010.

The letter, 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.  The news caused Halliburton stock to drop in value by almost 8 percent by the end of trading Thursday, to $31.68 a share.

Bartlit emphasized in his letter that cementing failures are a known hazard in the oil industry, with specific tests such as a “negative pressure test” and “cement evaluation logs” designed to identify cementing problems. However, he wrote, workers at BP and possibly the company that operated the Deepwater Horizon rig, Transocean, “misinterpreted or chose not to conduct such tests at the Macondo well.”

A commission hearing on the disaster is scheduled for November 9, but Bartlit’s letter said he was notifying the commission about the cement slurry issue immediately in order to “facilitate [its] consideration of their implications for offshore drilling safety.”