In a significant step toward stopping the worst offshore oil spill in U.S. history, BP said Wednesday mud that was forced down its blown-out well was holding back the flow of crude in the Gulf of Mexico and it was in a "static condition."
Workers stopped pumping mud in after about eight hours of their "static kill" procedure and were monitoring the well to ensure it remained stable, BP said.
Speaking to msnbc.com, BP spokeswoman Sheila Williams called the initial results "a very big deal."
"It's a milestone," Williams added. "It's a step toward the killing of the well." The next step would be deciding whether to cement the well, she said.
In a statement, BP said the well's pressure was being controlled by the mud pumped in on Tuesday, and that further pumping of mud "may or may not be required."
BP won't know for sure whether the static kill has succeeded until engineers can use a soon-to-be-completed relief well to check their work.
Speaking on NBC's TODAY, White House energy adviser Carol Browner said that the development suggested "we've turned an important corner here for people in the Gulf."
The static kill — also known as bullheading — involved slowly pumping the mud from a ship down lines running to the top of the ruptured well a mile below. BP has said that may be enough by itself to seal the well.
Before Wednesday's breakthrough, officials said the static kill effort would take 33 to 61 hours to complete.
But the mud that was forced down the broken wellhead to permanently plug the gusher is only half the story. To call the mission a success, crews working on a flotilla of vessels on a desolate patch of water need to seal off the well from two directions.
An 18,000-foot relief well BP has been drilling for the past three months will be used later this month to execute a "bottom kill," in which mud and cement will be injected into the bedrock 2½ miles below the sea floor to finish the job, retired Coast Guard Adm. Thad Allen said.
"There should be no ambiguity about that," Allen said. "I'm the national incident commander, and this is how this will be handled."
A 75-ton cap placed on the well in July has been keeping the oil bottled up inside over the past three weeks, but is considered only a temporary measure. BP and the Coast Guard want to plug up the hole with a column of heavy drilling mud and cement to seal it off more securely.
The task is becoming more urgent because peak hurricane season is just around the corner, Allen said. Tropical Storm Colin formed then dissipated far out in the Atlantic on Tuesday, but early forecasts say it will travel toward the East Coast rather than the Gulf.
The full magnitude of the Gulf of Mexico spill, triggered in April by a deadly explosion at the BP-owned Macondo well, became apparent on Monday as government scientists released revised figures showing almost 5 million barrels of oil leaked before the well was temporarily capped on July 15.
This made it the world's largest accidental maritime release of oil, surpassing the 1979 Ixtoc well blowout in Mexico's Bay of Campeche that gushed almost 3 million barrels.
Despite the breakthrough on the static kill operation, the new leak estimates could spell bad news for BP, which also faces an investigation by U.S. securities regulators into whether its employees profited illegally from the spill.
The revised flow numbers suggest the company had underestimated costs by at least $1 billion.
BP had estimated the well had leaked some 4 million barrels of oil and that it would be fined $1,100 per barrel under the Clean Water Act. The company faces fines of $4,300 per barrel if gross negligence is proven, but said it saw no need to change its provision as a result of the new estimate.
However, a New York Times report said there could be some more upbeat developments on Wednesday from a government report due to be released. It said the report would show three-quarters of the oil released has already evaporated, dispersed, captured or eliminated.