Crews hoped to begin pumping mud and perhaps cement down the throat of the blown-out oil well at the bottom of the Gulf of Mexico on Tuesday in what BP officials said could be the method of attack that finally snuffs the spill.

Engineers planned to probe the busted blowout preventer with an oil-like liquid to determine whether it could handle the static kill. If the test is successful, they plan to spend Tuesday through Thursday pumping the heavy mud down the well.

The so-called "static kill" is meant as insurance for the crews who have spent months fighting the oil spill. The only thing keeping oil from blowing into the Gulf at the moment is an experimental cap that has held for more than two weeks but was never meant to be permanent.

BP officials had insisted for months that a pair of costly relief wells were the only surefire way to kill the oil leak but said Monday that the static kill alone — involving lines running from a ship to the blown-out well a mile below — might do the trick.

BP Senior Vice President Kent Wells said that if the static kill is successful, the relief wells may not be needed to do the same thing weeks later, but from the bottom. The primary relief well, near completion, will still be finished and could be used simply to ensure the leak is plugged, he said.

"Even if we were to pump the cement from the top, we will still continue on with the relief well and confirm that the well is dead," he said. Either way, "we want to end up with cement in the bottom of the hole."

Government officials and company executives have long said the wells, which can cost about $100 million each, might be the only way to make certain the oil is contained to its vast undersea reservoir. A federal task force said about 172 million gallons of oil made it into the Gulf between April and mid-July, when a temporary cap bottled up all the oil.

The task force said actually about 206 million gallons total gushed out of the mile-deep well but a fleet of boats and other efforts were able to contain more than 33 million gallons.

The 172 million gallons is on the high end of recent estimates that anywhere from 92 million to 184 million gallons had gushed into the sea.

The company began drilling the primary, 18,000-foot relief well May 2, 12 days after the Deepwater Horizon rig exploded and killed 11 workers, and a second backup well May 16. The first well is now only about 100 feet from the target, and Wells said it could reach it as early as Aug. 11.

Retired Adm. Thad Allen, the government's point man on the spill response, said Monday that the focus now is on making sure the static kill is successful. But he cautioned that federal officials don't see it as "the end all, be all until we get the relief well done."

Before the effort can begin, engineers must probe the broken blowout preventer with an oil-like liquid to decide whether it can handle the static kill process. They had hoped to begin the hours-long test Monday but delayed it until Tuesday after a small leak was discovered in the hydraulic control system.

One of the biggest variables on the static kill's finality is whether the area called the annulus, which is between the inner piping and the outer casing, has sprung an oil leak. Engineers probably won't be able to answer that question until they drill in from the bottom, he said.

"Everyone would like to have this thing over as soon as possible," Allen said, adding: "We don't know the condition of the well until we start pushing mud into it."

The company's statements Monday might signal that it is more concerned than it has acknowledged about debris found in the relief well after it was briefly capped as Tropical Storm Bonnie passed last week, said Ed Overton, a Louisiana State University environmental sciences professor.

Plus, trying to seal the well from the top gives BP two shots at ending the disaster, Overton said.

"Frankly, if they can shut it off from the top and it's a good, permanent seal, I'll take it," Overton said. "A bird in the hand at this point is a good thing with this deal."

BP and federal officials have managed to contain large parts of the spill through skimmers, oil-absorbant boom and chemical dispersants meant to break up the oil.

Federal regulators have come under fire from critics who say that BP was allowed to use excessive amounts of the dispersants, but government officials counter that they have helped dramatically cut the use of the chemicals since late May.

The Environmental Protection Agency released a study Monday concluding that when mixed with oil, chemical dispersants used to break up the crude in the Gulf are no more toxic to aquatic life than oil alone.


Associated Press writers Jeffrey Collins and Harry R. Weber in New Orleans and Matthew Daly in Washington contributed to this report.