COVINGTON, La. -- BP is going in for the kill. The trick is to do the job quickly and cleanly.
As early as dawn today, the oil company will try to choke to death the gusher at the bottom of the sea by force-feeding it heavy drilling mud and cement -- a tactic called a "top kill" that is routinely used above ground but has never been tried 5,000 feet underwater.
If it's not done just right, it could make the leak worse.
The stakes for BP are high, with politicians and others losing patience with the company over its inability to stop the oil leak that sprang more than a month ago after an offshore drilling rig exploded. Eleven workers were killed, and by the most conservative estimate, 7 million gallons of crude have spilled into the Gulf of Mexico, fouling Louisiana's marshes and coating birds and other wildlife.
"We want what everybody wants -- to stop the flow at the source as quickly as possible," said BP spokesman John Curry. "We understand the frustration and we just want to bring this to closure."
Engineers were doing at least 12 hours of diagnostic tests Tuesday. They planned to check five spots on the well's crippled five-story blowout preventer to make sure it could withstand the heavy force of the mud. A weak spot in the device could blow under the pressure, causing a brand new leak.
BP has been drafting plans for the top kill for weeks but had to delay it several times as crews scrambled to assemble the equipment at the site 50 miles off the coast. A flotilla of rigs, barges and other heavy machinery stood ready there Tuesday.
A top kill has worked on aboveground oil wells in Kuwait and Iraq. BP CEO Tony Hayward pegged its chances of success in this case at 60 to 70 percent.
Kent Wells, a BP senior vice president, cautioned that engineers are speeding through a planning process that would normally take months. He warned that the top kill could be delayed or scuttled if Tuesday's pressure readings are bad.
Once the test results are in, scientists with the federal Minerals Management Service will examine them and BP will consult with government officials before deciding whether to press on, Curry said.
If all goes as planned, engineers will pump fluid twice as dense as water from two barges into two 3-inch-wide lines that will feed it into the blowout preventer. Crews plan to pump it in at a rate of 1,680 to 2,100 gallons per minute in hopes of counteracting the upward pressure of the oil gushing to the surface. They stockpiled some 50,000 barrels of the heavy mud, a manufactured substance that resembles clay.
Wells said it could take anywhere from a few hours to two days to determine whether the top kill is working.
If it succeeds, BP plans to follow through by injecting a stream of cement to permanently seal up the well. They may also install a new blowout preventer on top as a fail-safe.
Live video of the leak has been available online for the past few days, and BP said Tuesday that it will continue throughout the planned 'top kill' procedure. An Obama administration official, speaking on condition of anonymity to disclose private discussions, said BP agreed to keep the public video feed only under pressure from the administration.
Earlier in the day, Rep. Edward J. Markey, D-Mass., said he had learned that BP had planned to turn off the video feed during the top kill and complained that the blackout would "obscure a vital moment in this disaster."
Bob Bea, an engineering professor at the University of California at Berkeley, said the procedure carries a high risk of failure because of the velocity at which the oil may be spewing.
If some of the higher estimates of 3 million to 4 million gallons a day are correct, "it's going to spit everything back in your face," Bea told The Associated Press. He estimated that anything above 1.6 million gallons a day would be too much for a top kill to work.
Nevertheless, "they're trying and that's a good thing," Bea said. "I certainly pray that it works, because if it doesn't there's this long waiting time" before BP can dig relief wells that would cut off the flow.
In addition to the danger of the blowout preventer springing a leak, the risks include the possibility that the mud could tear a new hole in the leaking well pipe.
If the top kill doesn't work, or makes the problem worse, BP will probably turn to a containment box resting on the seafloor. It is a smaller version of the 100-ton box the company lowered several weeks ago in hopes of capturing much of the oil. That larger device was clogged with ice crystals and BP had to abandon it, but the company hopes the smaller version might work better.
BP has had limited success with a mile-long tube it installed more than a week ago to siphon up some of the oil. The device has captured more than 500,000 gallons but has also allowed untold amounts to escape into the sea.
The company's backup plans include a junk shot, which involves shooting golf balls, tire scraps, knotted rope and other assorted objects into the well to clog it up.
BP executives say the only guaranteed permanent solution is a pair of relief wells crews have already started drilling, but that will take at least two months.
Associated Press Writers Seth Borenstein and Erica Werner in Washington contributed to this story.