Also Thursday, a U.S. government source said the director of the federal agency charged with overseeing off-shore drilling had left her job in the wake of the April 20 spill, which appears to be the worst in the nation's history -- far bigger than the 11 million gallons that spilled in the Exxon Valdez disaster.
Elizabeth Birnbaum resigned "effective immediately" from the U.S. Minerals Management Service, the Interior Department agency that has been faulted for lax oversight in the wake of the mammoth spill, the government source said. It was not immediately known whether Birnbaum, who had been slated to testify on Capitol Hill today, had resigned of her own volition or been forced out.
What was clear was that the magnitude of the spill resulting from the explosion of a BP oil rig continues to grow, with potentially disastrous consequences.
U.S. Geological Survey Director Dr. Marcia McNutt said at a news conference Thursday that two teams of scientists, using different methods, have preliminarily determined that between 17 and 39 million gallons of oil have gushed into the ocean so far.
Desperate to stop the flow, BP engineers on Thursday launched an effort known as "top kill," pumping mud into the damaged blowout preventer in an effort to plug the leak. The hazardous-but-high-reward maneuver comes five weeks into the oil spill crisis amid an intensifying atmosphere of political recrimination that has spread from the Gulf Coast to the White House and Congress.
Early bulletins on the maneuver were encouraging. "The top kill procedure is going as planned, and it is moving along as everyone had hoped," Coast Guard Adm. Thad Allen, who has been leading the government's effort to end the leak, told CNN Thursday morning.
BP officials said that they hoped to know if the well was dead by Thursday evening, and Coast Guard Rear Admiral Mary Landry, the federal on-scene coordinator, said she was encouraged but reluctant to express optimism "until I know for sure that we've secured the well and the leak has stopped."
The billowing plumes of effluent from cracks in the top of the riser pipe no longer look like oil and gas but have a distinctly muddy appearance. "What you've been observing out of the top of that riser is most likely mud. We can't fully confirm that because we can't sample it," BP Chief Operating Officer Doug Suttles said at a news conference Wednesday night.
"It is a little like arm wrestling," BP managing director Bob Dudley told ABC's "Good Morning America," adding that the effort could last until Friday morning. "It is quite a titanic struggle of forces, and it's going to go slow."
On another front in the battle against the oil spill, the Coast Guard pulled 125 fishing vessels off the water in Breton Sound after fishermen who had been hired by BP to clean up the slick complained of nausea and chest pains, the Associated Press reported. This is the latest in a series of reports in recent weeks of fishermen feeling sick while trying to skim the oil.
President Obama, in a news conference Thursday, will outline tougher rules and regulations of the oil drilling industry and suspend exploratory drilling in the Arctic until at least next year, according to an administration official who spoke on condition of anonymity because the announcement had not been made. This will delay a controversial drilling effort by Shell this summer in the seas off northern Alaska. The moves come after a 30-day review of oil drilling that Obama ordered when the crisis began.