BP and its partners all made serious mistakes in the run-up to the largest offshore oil spill in the U.S. history, illustrating the need for a better safety culture in the oil drilling sector, White House oil spill commission co-chair Bill Reilly said on Tuesday.

Complacency at BP, as well as Transocean Ltd and Halliburton, led to serious missteps prior to the rig explosion that unleashed millions of barrels of oil into the Gulf of Mexico over the summer.

"BP, Halliburton and Transocean are major respected companies operating throughout the Gulf and the evidence is they are in need of top-to-bottom reform," Reilly, a former head of the Environmental Protection Agency, said at the start of the second session of commission's two-day meeting on the root causes of the spill this week.

Both Reilly and his commission co-chair Bob Graham sought to clarify comments made Monday by the commission's chief counsel that workers for the companies did not cut corners on safety to save money.

They said the panel's investigators did not find evidence that individual workers sacrificed safety over monetary concerns, but that does not mean that the companies involved placed enough emphasis on safety.

"The problem here is that there was a culture that did not promote safety...leaders did not take risks seriously enough, didn't identify risks that proved to be fatal," said Graham, a former U.S. Senator from Florida.

The panel's investigators detailed their preliminary findings about the causes of the BP spill on Monday, outlining a series of on-site misjudgments that preceded the drilling disaster that killed 11 people.

Representatives of BP and its partners were questioned by the panel's investigators for more than three hours at Monday's meeting. The companies disagreed on key aspects of responsibilities and causes of the spill.

President Barack Obama created the seven-member commission in the aftermath of the BP drilling accident. Its ultimate charge is to develop proposals to prevent and respond to major spills in the future.

The panel's final report and recommendations are scheduled to be released in January.