But the company, in a statement late Thursday, said responsibility for the explosion that killed 11 workers and set off the biggest oil spill in history lay firmly with BP for not testing the integrity of the cement job.

"BP, as the well owner and operator, decided not to run a cement bond log test even though the appropriate personnel and equipment were on the rig and available to run that test," Halliburton said.

The statement was issued after a presidential commission concluded that a faulty cement mix was at the root of the April 20 Deepwater Horizon explosion without explicitly blaming either BP or Halliburton.

The commission argued that a new cement foam containing nitrogen gas and other additives was pumped into the Macondo well before satisfactory tests proved the formula was stable.

"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," according to lead investigator Fred Bartlit.

"The cement should have prevented hydrocarbons from entering the well," Bartlit wrote in the panel's first findings presented to the seven members of the presidential commission in a letter Thursday.

The commission, which is investigating the roles of BP, Halliburton and drilling contractor Transocean, stressed that the cementing of the well casing on the sea floor was not the only factor that led to the disaster.

But Thursday's report was its first set of findings that could ultimately determine who is liable for an accident that killed 11 workers and is still impacting the fragile Gulf wetlands.

The commission found that three tests conducted by Halliburton showed its cement mix was unstable and that results from a fourth favorable test came in after cement was pumped in to secure the well casing.

But Halliburton revealed in its statement that, on orders from BP, it made a final change in the mixture from eight gallons of retarder to nine per 100 sacks of cement without conducting a so-called "stability test."

"Tests, including thickening time and compressive strength, were performed on the nine gallon formulation (the cement formulation actually pumped) and were shared with BP before the cementing job had begun," Halliburton said.

"A foam stability test was not conducted on the nine gallon formulation."

Halliburton had previously said it tested the cement before pumping it on April 19 and 20 and found it to be stable.

But the commission, tasked by US President Barack Obama to get at the root cause of the disaster, concluded that both Halliburton and BP knew weeks before the disaster that there were problems with the cement mix.

The commission had scientists conduct stability tests on cement made from what they believed were the same compounds as the slurry used on the BP well, but found it was unstable.

"This is like building a car when you know the brakes could fail, but you sell the cars anyway," said Congressman Edward Markey, chair of the energy and environment subcommittee in the House of Representatives' Energy and Commerce Committee.

"We now know what BP and Halliburton knew, and when they knew it. And we know they did absolutely nothing about it," Markey said.

"The fact that BP and Halliburton knew this cement job could fail only solidifies their liability and responsibility for this disaster."

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