BP began operations Tuesday to permanently plug the runaway well that has brought environmental and economic ruin to the Gulf of Mexico and spilled more oil into the sea than ever before.
Engineers launched their long-awaited static kill at 2000 GMT, ramming heavy fluids into the blown-out Macondo well to force the crude back down into a reservoir almost 3.5 miles (5.7 kilometers) beneath the surface of the sea.
Having conducted "text-book" tests that showed the oil could be subdued, BP was optimistic of success, although senior vice president Kent Wells said it was too early to know if the process would take hours or days.
Once the heavy drilling fluid, known as mud, is holding down the oil, the aim is to pour in a cement plug that will permanently seal off the reservoir.
Any leaks in the steel casing of the well would complicate matters as it would mean the area between the pipe and the outer well bore, known as the annulus, would also have to be filled up with mud.
The best case scenario could see the well put permanently out of action by Wednesday, although a "bottom kill" will be performed through a relief well in mid-August to cement in the outer well bore and make sure of success.
If the well casing has leaks, a decision could be taken to hold off on the cement job until the relief well is ready.