"What you've been observing coming out of the top of that riser is most likely mud," Doug Suttles, BP's chief operating officer, said at a news conference broadcast from a Louisiana command centre. "We can't fully confirm that because we can't sample it. And the way we know we've been successful is it stops flowing."
Residents along the Gulf Coast have been watching video of the leaking well since the "top kill" began about 1pm CDT (1800 GMT). The effort involves pumping tons of heavy drilling mud down the well bore to overcome the oil and gas flowing out. The mud is twice the weight of water, and it is hoped it will sink into the well to stop the massive flow of oil.