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Workers shovel oil from the Deepwater Horizon oil spill off Fourchon Beach in Port Fourchon, La., on Monday. ((Patrick Semansky/The Associated Press))

An internal investigation by oil giant BP suggests the massive oil spill in the Gulf of Mexico was caused by "an unprecedented combination of failures" of processes, systems and equipment.

"There were multiple control mechanisms — procedures and equipment — in place that should have prevented this accident or reduced the impact of the spill," BP said Monday night.

The London, England-based company has shared its investigation results with the U.S. Department of the Interior. But it stressed that the investigation was "a fact-finding effort" only that is by no means conclusive.

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BP chief executive Tony Hayward, center, speaks to reporters after observing cleanup efforts at Fourchon Beach in Port Fourchon, La., on Monday.

"I understand people want a simple answer about why this happened and who is to blame," CEO Tony Hayward said. "The honest truth is that this is a complex accident, caused by an unprecedented combination of failures.

"A number of companies are involved, including BP, and it is simply too early — and not up to us —  to say who is at fault."

The investigation focused on seven mechanisms, including the blowout preventer (BOP), a 408-tonne piece of equipment that sat on top of the wellhead during drilling operations.

A report by the Interior Department found staffers at the Minerals Management Service — which oversees offshore drilling — accepted tickets to sports events, lunches and other gifts from oil and gas companies.

Interior Secretary Ken Salazar called the report "deeply disturbing."

He said it highlights the need for reforms he has proposed, including a plan to abolish the minerals agency and replace it with three new entities.

The report "is further evidence of the cosy relationship between some elements of MMS and the oil and gas industry," Salazar said Tuesday.

"I appreciate and fully support the inspector general's strong work to root out the bad apples in MMS."

Several employees cited in the report have resigned, were fired, or referred for prosecution, he said.

— The Associated Press

The BOP has faced much scrutiny because its valves failed to close automatically after the rig exploded on April 20 and then sank. Oil leaks were found two days later. Since then, more than 27 million litres of crude have seeped from well site — about 80 kilometres off the coast of Louisiana —  into the Gulf.

'Top kill' in works

On Tuesday, BP said it was conducting extensive diagnostic tests to help it determine the best course of action in stopping the flow of oil from the well.

The diagnostics could take between 12 and 24 hours but could be delayed by "further issues," BP senior vice-president Kent Wells told reporters.

That means the earliest BP can attempt what it considers its best hope at stopping the flow — the "top kill" procedure — is Wednesday "and it could extend … on from there," Wells said.

The procedure involves pumping enough heavy drilling mud through pipes connected to the well to "overcome" the flow of oil and gas up the well and "cause it to go static," he said.

It could take anywhere from half a day to two days to complete the procedure, although Wells was careful to add that he wouldn't want people to think "it's more successful if it takes a shorter period of time."

As to the likelihood of its success, Wells said "there is always a possibility the top kill won't work."

He also warned that while the procedure has worked in other parts of the world, it's never been attempted at 1,500 metres below the water's surface, where the leaking pipe sits.