Published: Aug 04, 2010 10:37 AM EDT
Updated: Aug 04, 2010 7:32 AM EDT

ON THE GULF OF MEXICO (AP) - 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.

"It's a milestone," BP PLC spokeswoman Sheila Williams said. "It's a step toward the killing of the well."

The next step would be deciding whether to cement the well, Williams said. The pressure in the well dropped quickly in the first 90 minutes of the static kill procedure Tuesday, a sign that everything was going as planned, wellsite leader Bobby Bolton told The Associated Press. Bolton said Tuesday night that the procedure was going well.

"Pressure is down and appears to be stabilizing," he told the AP then.

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. 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. Before the cap was lowered onto the well, 172 million gallons of crude flowed into the sea, unleashed by the April 20 explosion aboard the drilling rig Deepwater Horizon that killed 11 workers.

A previous, similar effort failed in May when the mud couldn't overcome the unstemmed flow of oil. BP won't know for certain whether the static kill has succeeded until engineers can use the soon-to-be-completed relief well to check their work. 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.