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WASHINGTON (AP) — Early finger-pointing erupted Monday among companies involved in the oil rig explosion in the Gulf of Mexico and unstopped leak of millions of gallons of oil, on the eve of the first congressional hearings into the accident.

A top American executive for BP, Lamar McKay, said a critical safety device known as a blowout-preventer failed catastrophically. Separately, the owner of the rig off Louisiana's coast said BP managed it and was responsible for all work conducted at the site. A third company defended work that it performed on the deepwater oil well as "accepted industry practice" before last month's explosion.

"We are looking at why the blowout preventer did not work because that was to be the fail-safe in case of an accident," McKay, chairman and president of BP America, said in testimony prepared for a Senate hearing Tuesday. A copy of his testimony was obtained by The Associated Press. "Transocean's blowout preventer failed to operate."

The chief executive for Swiss-based Transocean, which owned the oil rig and the blowout preventer, shifted blame to BP.

"All offshore oil and gas production projects begin and end with the operator, in this case BP," CEO Steven Newman said in his Senate testimony, also obtained by the AP. Newman said BP was responsible for submitting a detailed plan specifying where and how a well is to be drilled, cased, cemented and completed.

Newman also said BP's contractor, Halliburton Inc., was responsible for encasing the well in cement, putting a temporary plug in the top of the well and ensuring the cement's integrity. That cementing process was dictated by BP's well plan, Newman said.

A Halliburton executive, Tim Probert, said the company safely finished a cementing operation 20 hours before the rig went up in flames. Probert said Halliburton completed work on the well according to accepted industry practice and at the direction of federal regulators.

The blame-game took hold on Capitol Hill as Congress and federal investigators were to begin a series of hearings in Washington and on the Gulf Coast. Two Senate hearings were set for Tuesday, and a House hearing was scheduled for Wednesday. In Louisiana, near the disaster site, a six-member panel that includes investigators from the Interior Department and Coast Guard was to begin two days of hearings.

McKay, the BP executive, said the company wants answers itself. He disclosed that the company has at least 40 people internally investigating the accident, but he acknowledged that the cause is still a mystery. Transocean has its own investigative team, Newman said.

"We are looking at our own actions and those of our contractors," McKay wrote in his Senate testimony.

Newman said it makes no sense to suggest the blowout preventer caused the accident. He said it was ironic that attention was being focused on the blowout preventer because at the time of the explosion drilling at the site was finished.

The blowout preventer, made by Houston-based Cameron Inc., is a 450-ton piece of equipment that sits on top of the wellhead during drilling operations. It contains valves that can be closed remotely in case of an accident or increase in pressure.

"The systems are intended to be fail-safe; sadly and for reasons we do not yet understand, in this case, they were not," McKay said.

The cause of the explosion is under investigation, but lawsuits filed after the disaster have alleged it occurred when Halliburton workers improperly capped the well — a process known as cementing. Halliburton denies wrongdoing.

According to a 2007 study by the Minerals Management Service, an agency within the Interior Department, cementing was a factor in 18 of 39 rig blowouts in the gulf between 1992 and 2006.

Probert of Halliburton said the company had four workers stationed on the rig performing several tasks, including cementing — a process of applying cement and water to a pipe to prevent the wall of the hole from collapsing during drilling.

A positive pressure test was conducted after the work was finished to demonstrate the integrity of the cement job, Probert said. Transocean, the well owner, decided to continue operating the well after the cement job was completed, he said.

Probert said a "negative" pressure test, which measures the integrity of the casing seal assembly, was conducted by BP at the direction of Transocean and according to requirements of the Minerals Management Service.

After the test was completed, BP "then continued to displace the riser with seawater before the planned placement of the final cement plug, which would have been installed inside the production string and enabled the planned temporary abandonment of the well," Probert said.

Before Halliburton personnel could set the final cement plug, the explosion occurred. "As a result, the final cement plug was never set," Probert said.

David Nagel, a senior vice president and head of BP America's Washington office, emphasized in a meeting with reporters Monday that the company is making every effort to find a way to halt the spill, pay for cleanup and address claims from people that are being harmed — a theme that will be repeated by BP executives to senators.

"We want to do the right thing," Nagel said.

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Associated Press writers H. Josef Hebert and Frederic J. Frommer contributed to this report.