WASHINGTON — The blame game is in full throttle as Congress begins hearings on the massive oil spill threatening sensitive marshes and marine life along the Gulf Coast.

Executives of the three companies involved in the drilling activities that unleashed the environmental crisis are trying to shift responsibility to each other in testimony to be given at separate hearings Tuesday before two Senate committees, even as the cause of the rig explosion and spill has yet to be determined.

Lawmakers are expected to ask oil industry giant BP, which operated the Deepwater Horizon drilling rig 40 miles off the Louisiana coast, why its drilling plans discounted the risk that such a catastrophic pipeline rupture would ever happen, and why it assumed that if a leak did occur, the oil would not pose a major threat.

The morning hearing by the Energy and Natural Resources Committee and the afternoon session before the Environmental and Public Health Committee give lawmakers their first chance to question the executives publicly about the April 20 rig fire, attempts to stop the flow of oil and efforts to reduce the damage.

Copies of planned testimony, obtained Monday by The Associated Press, brought into the open fissures among the companies caught up in the accident and its legal and economic fallout.

A top executive of BP PLC, which leased the rig for exploratory drilling, focuses on a critical safety device that was supposed to shut off oil flow on the ocean floor in the event of a well blowout but "failed to operate."

"That was to be the fail-safe in case of an accident," Lamar McKay, chairman of BP America, says, pointedly noting that the 450-ton blowout protector — as well as the rig itself — was owned by Transocean Ltd.

Of the 126 people on the Deepwater Horizon rig when it was engulfed in flames, only seven were BP employees, said McKay.

But Transocean CEO Steven Newman was seeking to put responsibility on BP.

"Offshore oil and gas production projects begin and end with the operator, in this case BP," said Newman, according to the prepared remarks. His testimony says it was BP that prepared the drilling plan and was in charge when the drilling concluded and the crew was preparing to cap the well 5,000 feet beneath the sea.

To blame the blowout protecters "simply makes no sense" because there is "no reason to believe" that the equipment was not operational, Newman argues.

Newman also cites a third company, Halliburton Inc., which as a subcontractor was encasing the well pipe in cement before plugging it — a process dictated by BP's drilling plan.

A Halliburton executive, Tim Probert, planned to assert that the company's work was finished "in accordance with the requirements" set out by BP and with accepted industry practices. He says pressure tests were conducted after the cementing work was finished to demonstrate well integrity.

BP and Transocean are conducting separate investigations into what went wrong.

In Louisiana, the Coast Guard and the Interior Department's Minerals Management Service were beginning two days of hearings on the cause of the explosion. The list of witnesses scheduled to testify includes a Coast Guard search and rescue specialist, crew members from a cargo vessel that was tethered to the Deepwater Horizon rig and two Interior inspectors.

In other developments:

— Interior Secretary Ken Salazar will propose splitting up the Minerals Management Service, an administration official, who asked not to be identified because the plan is not yet public, told The Associated Press. One agency would be charged with inspecting oil rigs, investigating oil companies and enforcing safety regulations, while the other would oversee leases for drilling and collection of billions of dollars in royalties.

— Homeland Security Secretary Janet Napolitano and Alabama Gov. Bob Riley will tour the Mobile Incident Command Center in Mobile, Ala., on Tuesday.

— The Environmental Protection Agency gave the go-ahead Monday to use oil dispersing chemicals near the sea bottom where the oil is leaking, although the agency acknowledged ecological effects of the chemical are not yet fully known. Two tests have shown the procedure helps break up the oil before it reaches the surface.

— BP said it has spent $350 million so far on spill response activities.

— President Barack Obama, after being briefed on the latest developments Monday, directed that more independent scientists get involved in seeking a solution to the spill. Energy Secretary Steven Chu will take a team of scientists to BP in Houston.

— BP said it has received 4,700 claims for damages related to the spill and so far has paid out $3.5 million on 295 of the claims.