Halliburton Co. knew the cement mixture it used in BP PLC's ill-fated well in the Gulf of Mexico was unstable two months before the April 20 blowout that triggered the worst oil spill in U.S. history, according to the staff of the presidential commission investigating the spill.

Documents from Halliburton show that before it started pumping cement into the wellbore on April 19 and 20, the company knew the results of three separate laboratory tests on the cement showed that the mixture did not meet industry standards, the commission's staff wrote today in a letter to the seven-member panel.

Halliburton provided BP with the data from at least one of the tests in March, but "there is no indication that Halliburton highlighted to BP the significance of the foam stability data or that BP personnel raised any questions about it," Fred Bartlit Jr., chief counsel for the commission, wrote.

One of four cement tests conducted in the months leading up to the disaster indicated the mixture would be stable, Bartlit said. And details remain uncertain about a final test conducted in the days before the blowout.

"We are not yet certain whether Halliburton reported this data internally or whether the test was even complete prior to the time the cement job was poured at the Macondo well," the letter says.

Bartlit does not fault the cement job as the primary or sole cause of the blowout that killed 11 rig workers and sparked the oil spill. But he is clear that if the cement had worked properly and kept the pressurized oil and gas out of the wellbore, the blowout could have been prevented.

"We have known for some time that the cement used to secure the production casing and isolate the hydrocarbon zone at the bottom of the Macondo well must have failed in some manner," Bartlit wrote. "The cement should have prevented hydrocarbons from entering the well."

BP has also faulted Halliburton's role in the disaster (Greenwire, Sept. 8). But Halliburton officials have said that the company properly tested and used the cement mixture in the BP well and that BP's flawed well design and operations are to blame.

A Halliburton spokeswoman today said the company is still reviewing the commission's letter.

Rep. Ed Markey (D-Mass.) blasted both the companies on the news and used the opportunity to continue his push for BP CEO Bob Dudley to testify before his House Energy and Commerce Subcommittee on Energy and Environment and to plug a bill he introduced that would grant the commission subpoena power.

"The fact that BP and Halliburton knew this cement job could fail only solidifies their liability and responsibility for this disaster," Markey said in a statement. "This is like building a car when you know the brakes could fail, but you sell the cars anyway."

The commission meets next month to discuss the panel's preliminary findings.

Click here to read the letter.