If there’s one thing Halliburton doesn’t need, it’s a blow to their reputation. They had such a sterling record up to now!

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 panel’s 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.

Whether or not this could lead to criminal negligence charges is unclear. But it certainly could have a bearing on the civil penalties. Halliburton and the other companies involved, like Transocean, have been pointing the finger at BP, and vice versa. At stake is responsibility for the tens in billions in expected fines for violating the Clean Water Act and other environmental statutes, as well as claims from individuals seeking damages. That claims process sounds like it’s going about as well as HAMP, by the way. But for months now, the question has been who will pay. This seems like incontrovertible evidence that Halliburton shares at least some of the liability. However, not only may their contract keep them safe, but the addition of multiple actors’ responsibility for the spill may lower the ultimate fine.

Halliburton’s contract makes it unlikely that Halliburton will face much liability for the disaster, said Matthew Conlan, an analyst for Wells Fargo Securities. But the latest revelations could hurt Halliburton’s reputation, he added. “The integrity of their product is being questioned and the integrity of their advice is being questioned.”

BP could benefit if investigators determine that Halliburton’s cement design was faulty, experts said.

Under federal pollution laws, BP will face much higher penalties if it is found to have been “grossly negligent” in the spill. Such a finding is less likely if several different companies share the blame.

Certainly the markets believe that Halliburton will take a hit, either out of the pocketbook or to their reputation; they tanked in trading yesterday, down 8%. Couldn’t happen to a bigger caricature of faceless corporatism. But the fact that two companies failing means a smaller overall fine than one company failing suggests a real problem with the law.

The lead investigator’s law firm has done work for Halliburton prior to 2005, incidentally. Those corporate tentacles are long and widespread.