WASHINGTON  Halliburton, whose failed cement job on the BP well in the Gulf of Mexico was identified as a contributing factor to the deadly blowout by a presidential investigative panel on Thursday, is defending its work and assigning the blame for the accident to BP.

In a six-page statement issued Thursday night, Halliburton questioned tests that showed its cement mixture to be unstable and incapable of holding back the oil and the gas in the well, saying the tests were conducted on formulas other than what was eventually used on BP’s Macondo well. It said that a sample of the cement mixture it planned to use on the well, tested shortly before pumping began on April 19, had produced a positive result.

But Halliburton admitted that no stability test was conducted on the actual recipe for the cement used on the well. The company said that BP had ordered a change in Halliburton’s customary formula for cement by adding a higher proportion of a chemical that slows the hardening of the mixture.

The well blew out on April 20, killing 11 workers and eventually releasing nearly five million barrels of oil into the gulf. Since then, BP, Halliburton, Transocean and other partners in the well have traded accusations of blame as civil and criminal investigations have proceeded.