BP says its engineers have begun pumping heavy drilling mud into the blown-out Gulf of Mexico oil well in hopes of choking it for good.
BP spokesman John Barnes says crews launched the so-called "static kill" process Tuesday at 3 p.m. Central time to plug up the well and then possibly seal it with cement.
The well has leaked millions of gallons of oil into the Gulf since a BP rig exploded in April and killed 11 workers. The company was able to bottle up the leak last month with a cap, but engineers always considered that a temporary solution.
BP engineers say the static kill may permanently plug the well but they may not know for sure until they finish a relief well later this month.
They then plan to pump mud and cement through that to permanently suffocate the source of the oil.
"This is a really positive step forward," retired Coast Guard Adm. Thad Allen said earlier of the pending static kill. "It's going to be good news in a time where that hasn't been very much good news, but it shouldn't be a cause for premature celebration."
Special Section: Gulf Coast Oil Disaster
The static kill is meant as insurance for the crews that have spent months fighting the 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 now have said that if the static kill is successful, the well could be permanently sealed without them, reports CBS News correspondent Don Teague.
The company said there's no way to tell until the relief wells engineers have been digging for three months is complete.
Allen, the government's point man on the spill response, added earlier Tuesday that there "should be no ambiguity" that the primary relief well - which could be completed as early as Aug. 11 - will be finished, regardless. And the only surefire way to make certain the well is permanently plugged is to later fill it from below with mud and cement in a so-called "bottom kill," he said.
The testing was supposed to be completed Monday, but a minor leak discovered in the hydraulic control system pushed back the diagnostics until Tuesday. Allen said leaks have been repaired.
While the static kill could take days to complete, mostly because it involves the slow pumping of mud, Allen said crews should know within hours of its start whether the process is working.
It's important to begin soon, he said, with the peak hurricane season just around the corner. Tropical Storm Colin formed far out in the Atlantic on Tuesday, but early forecasts put it on a track off the East Coast rather than the Gulf.
And while diagnostic tests show that the 75-ton cap that has bottled up the oil since mid-July is sound, the static kill would give scientists more confidence the well won't leak again, he said.
"The quicker we get this done, the quicker we can reduce the risk of some type of internal failure" of the massive cap, he said.
A federal task force said Monday that about 172 million gallons (650 million liters) of oil made it into the Gulf between April and July 15, when the temporary cap contained all the oil.
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Judging by the latest estimate, BP could be fined up to $5.4 billion under the Clean Water Act, or as much as $21 billion if it is found to have committed gross negligence or willful misconduct.
The high-end fine would drop to around $17.6 billion if the government credits BP for the oil it has recovered, while the low-end fine would be around $4.5 billion.
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Any fines would be on top of the compensation BP has agreed to pay to thousands of people harmed by the spill. Under pressure from the White House, the company set up a $20 billion escrow fund to pay all claims, including environmental damages and state and local response costs.
The company began drilling the primary, 18,000-foot (5,500-meter) 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 (30 meters) from the target and is expected to be completed later this month.
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As businesses along the coast continued to clamor for relief from losses caused by the spill, BP said it created a special team to reduce paperwork and speed up payments to "businesspeople who are suffering."
