"This is not the end, but it will virtually assure us that there will be no chance of oil leaking into the environment," Allen.
If the mud plug in the blown-out well is successfully augmented with the cement, the next step involves an 18,000-foot relief well that intersects with the old well just above the undersea reservoir that had been losing oil freely since the Deepwater Horizon rig exploded off Louisiana on April 20, killing 11 workers.
Federal officials and BP executives won't declare the threat dashed until they use the relief well, despite the progress on the static kill.
"We have always said that we will move forward with the relief well. That will be the ultimate solution," BP Senior Vice President Kent Wells said Wednesday afternoon. "We need to take each step at a time. Clearly we need to pump cement. If we do it from the top, we might alter what we do with the relief well, but the relief well is still a part of the solution. The ultimate objective is getting this well permanently sealed."
Allen said Tuesday that to be safe, the gusher will have to be plugged up from two directions, with the relief well being used for the so-called "bottom kill."
According to a report released Wednesday by the Interior Department and the National Oceanic and Atmospheric Administration, there's still oil in the Gulf or on its shores - nearly 53 million gallons of it. That's still nearly five times the size of the Exxon Valdez spill, which wreaked environmental havoc in Alaska in 1989.
Fishermen boaters, swimmers and scuba divers continue to find oil and tar balls in areas that have been declared clear.
But almost three-quarters of the nearly 207 million gallons of oil that leaked overall has been collected at the well by a temporary containment cap, been cleaned up or chemically dispersed, or naturally deteriorated, evaporated or dissolved, the report said.
- The remaining oil, much of it below the surface, remains a threat to sea life and Gulf Coast marshes, NOAA Administrator Jane Lubchenco said. But the spill no longer threatens the Florida Keys or the East Coast, according to the Interior Department and the National Oceanic and Atmospheric Administration.
- President Barack Obama, while noting that people's lives "have been turned upside down," declared that the operation was "finally close to coming to an end."
- BP shares rose 2.7 percent to $40.46 in morning trading in New York. At one point they reached $40.75, their highest level since May 28.
The Associated Press contributed to this story.