Scientists declared the five-week-old BP spill in the Gulf of Mexico to be the worst in U.S. history on Thursday, while federal and oil industry officials capped a day of confusion by announcing they had suspended their mud-pumping "top kill" in its 10th hour before resuming Thursday night.
A 10-hour burst of 15,000 barrels of mud on Wednesday slowed the spill, said BP's chief operating officer Doug Suttles. But engineers suspended it to replenish the mud and review procedures.
Thursday evening, BP said it had resumed pumping. Officials said it could be late today or the weekend before the company knows if it has cut off the oil that has been flowing for five weeks.
In Washington, President Barack Obama announced major new restrictions on drilling projects, and the head of the federal agency that regulates the industry resigned under pressure, becoming the highest-ranking political casualty of the crisis so far.
As hour after hour passed after the top kill began Wednesday afternoon, technicians along with millions of television and Internet viewers watched live video images showing that the dark oil escaping into the gulf waters was giving way to a mud-colored plume.
That seemed to be an indication that the heavy liquids known as "drilling mud" were filling the chambers of the blowout preventer, replacing the escaping oil.
In the morning, federal officials expressed optimism that all was going well.
"The top kill procedure is going as planned, and it is moving along as everyone had hoped," Adm. Thad W. Allen of the Coast Guard, the leader of the government effort, told CNN.
It was not until late afternoon that BP acknowledged that the operation was not succeeding and that pumping had halted at 11 p.m. Wednesday.
"We have not yet stopped the flow, so the operation has not achieved its objective," Suttles said, adding that "nothing has gone wrong or has been unanticipated."
The top kill is the latest in a string of attempts to stop the oil that has been spewing since the drilling rig Deepwater Horizon exploded April 20. Eleven workers were killed.
If the procedure works, BP will inject cement into the well to seal it permanently. If it doesn't, the company has a number of backup plans. Either way, crews will continue to drill two relief wells, considered the only surefire way to stop the leak.
The U.S. Geological Survey disclosed a new series of studies that found the leak was a magnitude of two to five times larger than initial estimates.
Two teams of scientists calculated the well has been spewing between 504,000 and more than a million gallons a day. Even using the most conservative estimate, that means about 18 million gallons have spilled so far. In the worst-case scenario, 39 million gallons have leaked.
That larger figure would be nearly four times the size of the Exxon Valdez disaster, in which a tanker ran aground in Alaska in 1989, spilling nearly 11 million gallons.
"It's as if two Exxon Valdez tankers have already run aground and more are on the way if they don't get this hole plugged," said Jeremy Symons, senior vice president of the National Wildlife Federation. "Now we know the true scale of the monster we are fighting in the gulf. BP has unleashed an unstoppable force of appalling proportions."
BP officials said that the previous estimate of 210,000 gallons a day was based on the best data available at the time and that the company's response was not tied to the estimate.
"I don't believe at any time we have misled anybody on this," Suttles said.
The spill is not the biggest ever in the gulf. In 1979, a drilling rig in Mexican waters — the Ixtoc I — blew up, releasing 140 million gallons of oil.
In Washington, Elizabeth Birnbaum stepped down as director of the Minerals Management Service, a job she had held since July. Her agency has been harshly criticized over lax oversight of drilling and cozy ties with industry.
An internal Interior Department report released earlier this week found that between 2000 and 2008, agency staff members accepted tickets to sports events, lunches and other gifts from oil and gas companies and used government computers to view pornography.
Polls show the public is souring on the administration's handling of the catastrophe, and Obama sought to assure Americans that the government is in control and deflect criticism that his administration has left BP in charge.
Obama said he would end the "scandalously close relationship" between regulators and the oil companies they oversee. He also extended a freeze on new deepwater oil drilling and canceled or delayed proposed lease sales in the waters off Alaska and Virginia and along the Gulf Coast.
Obama said he was wrong to assume that oil companies were prepared for the worst as he tried to expand offshore drilling. His team did not move with "sufficient urgency" to reform regulation of the industry. In dealing with BP, his administration "should have pushed them sooner" to provide images of the leak, and "it took too long for us" to measure the size of the spill.
"In case you're wondering who's responsible, I take responsibility," Obama said as he concluded the news conference. "It is my job to make sure that everything is done to shut this down."
Meanwhile, officials in Pensacola reported about 100 suspected tar balls picked up on Pensacola Beach and another 100 on nearby Perdido Key this week. They have been shipped to a laboratory to determine whether the tar balls are made of oil from Deepwater Horizon. A similar group of tar balls that landed in the Keys recently turned out to be from an unrelated source, and so far no oil from the month-old disaster has turned up on a Florida beach.
Also Thursday, scientist said a dramatic change in the gulf's loop current has trapped a slick of oil in a huge circular eddy that appears likely to push slowly west instead of pumping the oil south into the Florida Keys.
The shift has at the least reduced the imminent environmental threat for Florida. Tar balls predicted to be floating in the Florida Straits by now instead might not arrive for weeks, months or — depending on lots of variables — maybe at all.
Information from McClatchy Newspapers, the Associated Press and the New York Times was used in this report.