WASHINGTON Ñ Halliburton knew weeks before the fatal explosion of the Macondo well in the Gulf of Mexico that the cement mixture they planned to use to seal the bottom of the well was unstable but still went ahead with the job, the presidential commission investigating the accident said on Thursday.

In the first official finding of responsibility for the blowout, which killed 11 workers and led to the largest offshore oil spill in American history, the commission staff determined that Halliburton had conducted three laboratory tests that indicated that the cement mixture did not meet industry standards.

The result of at least one of those tests was given on March 8 to BP, which failed to act upon it, the panelÕs lead investigator, Fred H. Bartlit Jr., said in a letter delivered to the commissioners on Thursday.

Another Halliburton cement test, carried out about a week before the blowout of the well on April 20, also found the mixture to be unstable, yet those findings were never sent to BP, Mr. Bartlit found.

Although Mr. Bartlit does not specifically identify the cement failure as the sole or even primary cause of the blowout, he makes clear in his letter that if the cement had done its job and kept the highly pressured oil and gas out of the well bore, there would not have been an accident.

ÒWe 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,Ó he said in his letter to the seven members of the presidential commission. ÒThe cement should have prevented hydrocarbons from entering the well.Ó

The failure of the cement set off a complex and ultimately deadly cascade of events as oil and gas exploded upward from the 18,000-foot-deep well. The blowout preventer, which sits on the ocean floor atop the well and is supposed to contain a well bore blowout, also failed.

In an internal investigation, BP identified the faulty cement job as one of the main factors contributing to the accident and blamed Halliburton, the cementing contractor on the Macondo well, as the responsible party. Halliburton has said in public testimony that it tested and used a proper cement formula on the well and that BPÕs flawed well design and poor operations caused the disaster.

Jesse Gagliano, a Halliburton technical adviser, told federal investigators in Houston in August that the company was confident in the cement job. He said Halliburton initially recommended that BP use a well design with more devices called Òcentralizers.Ó But even after BP chose to use six centralizers instead of the recommended 21, Halliburton believed the cement would work properly, Mr. Gagliano said.

ÒAll indication at the surface was that everything went fine,Ó he said. ÒWe plan for success.Ó

The Deepwater Horizon drilling rig was operated by a third company, Transocean.

Cathy Mann, a Halliburton spokesperson, said the company was reviewing the panelÕs findings and would provide comment later Thursday.

Halliburton, a major oil field services company and one of the nationÕs largest defense contractors, was once led by former Vice President Dick Cheney. Mr. BartlitÕs law firm, Bartlit Beck Herman Palenchar & Scott, has done legal work for Halliburton in the past but has not represented the company since 2005, the firm said.

After the commission report was released at 1:30 p.m. Eastern time, HalliburtonÕs stock price began dropping sharply, and was down more than 10 percent in mid-afternoon trading. BP shares rose slightly.

The commission obtained from Halliburton samples of the same cement recipe used on the failed well, including the same proportion of nitrogen used as a leavening agent and a number of chemicals used to stabilize the mixture. The cement slurry was sent to a laboratory owned by Chevron for independent testing.

The mixture failed nine separate stability tests designed to reproduce conditions at the BP well and did not pass any, according to ChevronÕs test results, which were returned to the commission this week.