WASHINGTON  Halliburton officials knew weeks before the fatal explosion of the BP 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 biggest 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 panel’s lead investigator, Fred H. Bartlit Jr., said in a letter delivered to the commissioners on Thursday. “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,” Mr. Bartlit said in his report.

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, meaning it was unlikely to set properly in the well, but those findings were never sent to BP, Mr. Bartlit found after reviewing previously undisclosed documents.