BP's Deepwater Horizon well was gushing oil at full force into the Gulf of Mexico as engineers began a high-stakes operation to remove a leaking cap, and install a more effective containment system.
The success of the operation is critical for BP and the White House, which pushed the oil company to switch its containment systems.
Barack Obama faces an additional drama tomorrow when a presidential commission appointed to determine the causes of the disaster holds its first public hearing in New Orleans. Oil industry advocates have accused the commission of bias.
Ghostly images from BP's deep-sea cameras showed robot arms executing the first stage of an anticipated week-long operation: lifting off a flange that had been attached to the well by six 24kg (52lb) bolts. "Everything is starting to come together," Kent Wells, a senior BP vice-president, said.
If all goes well, the switch to a tighter cap will allow the collection of all the crude oil spewing into the Gulf. The process could take three to six days.
But until then the containment effort that had been collecting some 15,000 barrels a day is suspended. The well is releasing 60,000 barrels of oil a day, and the leak that started in April has become America's worst environmental disaster.
Wells said it was impossible to guarantee success. "We have tried to work out as many of the bugs as we can," he said. "The challenge will come with something unexpected."
As operations continued, members of the presidential commission fanned out across the Gulf, meeting local people to offer assurances about their mission.
The commission is charged with determining the causes of the disaster and reviewing industry safety standards and government regulations. William Reilly, the commission's co-chair, who led the Environmental Protection Agency under President George Bush Sr, said he also wants to look at the response to the spill.
"Why were some of the decisions made with respect to both regulation and to immediate response?" he asked. "They look irregular to the casual observer."
The commission is to hold its first public hearings tomorrow, when it will hear from Coast Guard officials and BP executives about the progress of the clean-up effort.
But its members have already come under attack from the oil industry and supporters of offshore drilling.
The Wall Street Journal and the New Orleans Times-Picayune have accused Obama of stacking the seven-member commission with environmentalists rather than petroleum engineers.
"The president weighted the group with experts who appear more qualified to deal with the spill's effects than with its causes," the Times-Picayune said.
The Senate has also sought to undercut the authority of Obama's commission, with the energy and national resources committee voting to create a separate congressional investigation.
But Don Boesch, a marine science professor at the University of Maryland and a commission member, dismissed suggestions of bias. He said the commission would be focused on determining the causes of the disaster and making policy recommendations – not weighing in on the future of offshore drilling.
"Drilling is going to be a big part of the economic base of this region as well as the US energy supply for some time to come. We recognise that," he said. "It is the president and the Congress that makes those decisions about where we do offshore and gas development."
He also rejected the argument that the commission needed more petroleum engineers, noting that it had a former Shell executive as a scientific adviser.
"We are not mechanics who lift up the hood and try and figure out the details of the machinery," he said. "We are looking at the whole car and seeing how to make it run better." The operation now under way on the ocean floor will involve installing a cap weighing about 70 tonnes above the well. The cap will eventually allow three or four vessels to hook up and begin collecting oil through flexible hoses. The equipment can be disconnected much more readily than now, an important consideration in hurricane season.
The stronger seal could also help preparations for what is seen as the ultimate solution to the gusher in the Gulf – smothering it by pumping in heavy drilling fluid from a relief well.