The disk, part of the subsea safety infrastructure, may have ruptured during the surge of oil and gas up the well on April 20 that led to the explosion aboard the Deepwater Horizon rig, BP officials said. The rig sank two days later, triggering a leak that has since become the worst in U.S. history.
The broken disk may have prevented the heavy drilling mud injected into the well last week from getting far enough down the well to overcome the pressure from the escaping oil and gas, people familiar with BP's findings said. They said much of the drilling mud may also have escaped from the well into the rock formation outside the wellbore.
As a result, BP wasn't able to get sufficient pressure to keep the oil and gas at bay. If they had been able to build up sufficient pressure, the company had hoped to pump in cement and seal off the well. The effort was deemed a failure on Saturday.
BP started the top-kill effort Wednesday afternoon, shooting heavy drilling fluids into the broken valve known as a blowout preventer. The mud was driven by a 30,000 horsepower pump installed on a ship at the surface. But it was clear from the start that a lot of the "kill mud" was leaking out instead of going down into the well.
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BP tried to get around that problem with a series of "junk shots," in which materials like shredded rubber tires, pieces of rope and golf balls were fired in to clog holes in the valve. BP said that despite pumping more than 30,000 barrels of mud in three attempts at rates of as much as 80 barrels a minute, the operation "did not overcome the flow from the well."
The administration told BP on Saturday to halt the top-kill procedure, after becoming "very concerned" that the operation was putting too much pressure on the out-of-control well.
With the failure of its "top kill" effort, BP has begun the risky step of cutting off a bent pipe into the well and trying to cap it.
Coast Guard Adm. Thad Allen, the national incident commander, said Tuesday that BP was making its first major cut with super sheers that weigh about 46,000 pounds and resemble a giant garden tool. The company will also use a powerful diamond-edged cutter to try to make a clean cut above the blowout preventer, then will lower a cap over it with a rubber seal.
It could be as many as three days before the oil can be siphoned to the surface, Adm. Allen said.
Carol Browner, a top adviser to President Barack Obama, said she doesn't want to guess the prospects for success when BP tries to use the containment cap. Interviewed Tuesday on ABC's "Good Morning America," the White House energy and global warming czar said, "I don't want to put odds on it. ... We want to get this thing contained."
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Ms. Browner said "everyone, I think, is hoping for the best, but we continue to plan for the worst." She said she's concerned about the impact the hurricane season could have on ending the environmental crisis.
Meanwhile, "Avatar" and "Titanic" director, James Cameron, joined a group of scientists and other experts who met Tuesday with officials from the Environmental Protection Agency and other federal agencies for a brainstorming session on stopping the massive oil leak.
The Canadian-born director is considered an expert on underwater filming and remote vehicle technologies.
Meanwhile, the National Oceanic and Atmospheric Administration again expanded the area of the Gulf that is closed to fishing because of the spill.
NOAA said its announcement Tuesday coincides with the opening of the Gulf of Mexico recreational red snapper season and will affect some areas targeted by charter boat captains and private anglers. It kept the door open on possibly extending the end of the season, which is set to close July 24.
The closed area now covers 75,920 square miles, or slightly more than 31% of federal waters in the Gulf. It was extended from nearly 26% of the Gulf to include a slick moving into waters off eastern Alabama and the western tip of the Florida panhandle, as well as some large patches of sheen moving onto the west Florida shelf and southward to Cuban waters.
NOAA has banned fishing in these areas to ensure that seafood from the Gulf will remain safe for consumers. It is working with the Food and Drug Administration to implement a broad seafood sampling plan, which includes sampling seafood from inside and outside the closure area and at docksides and markets.
—Joan E. Solsman and the Associated Press contributed to this article.
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