The presidential commission investigating the BP oil spill has reached a stark conclusion about a factor that contributed to the deadly April drilling explosion:  The cement used to seal the bottom of the well was faulty. Moreover, cement contractor Halliburton Co. and BP both knew it.

Commission investigator Fred H. Bartlit Jr. did not identify the cement failure as the sole or even the main cause of the disaster, according to The New York Times. But he made clear that the accident would have been avoided had the cement done its job.

The commission found that, in the weeks prior to the blast, Halliburton conducted three laboratory tests that indicated the cement mixture did not meet industry standards. On March 8, BP was given the results of at least one of the tests, the Times said.

More testing was conducted by Halliburton shortly before the defective cement was poured on April 19, a day before the blowout occurred. However, the commission concluded, “Halliburton may not have had — and BP did not have — the results of that test before the evening of April 19, meaning that the cement job may have been pumped without any lab results indicating that the foam cement slurry would be stable.”

The failure of the cement allowed highly pressured oil and gas to enter the drill hole and spew upward from the 18,000-foot-deep well. The blowout preventer that sits atop the well also failed. Eleven workers were killed, and close to 200 million gallons of oil gushed in the Gulf of Mexico in the ensuing weeks. The leak was not stopped until July 15, and only this September, when a permanent cement plug was put in place, did officials declare the well dead.

In an internal investigation, BP found that faulty cement was one of the major contributors to the accident, though they ultimately pointed the finger at Halliburton.

In public testimony, Halliburton has said “BP’s flawed well design and poor operations” caused the disaster.

According to the Times, more than a gallon of the actual cement mixture used on the BP well is being held as evidence as both criminal and civil investigations continue.