BP Oil Spill Called Worst in U.S. History, as MMS Official Steps Down

BP once again has resumed its "top kill" operation, the company's latest effort to stem the flow of oil into the Gulf of Mexico in what appears to be the worst oil spill in U.S. history based on new estimates of the oil flow released today by a government task force.

BP earlier said that it halted the top kill operation late Wednesday evening. The company said it was not a sign of a problem with the procedure, but that it was pausing to assess the effectiveness of its pumping of drilling fluid into the well. It could take 24 to 48 hours to determine whether it has worked.

The process resumed at around 6 p.m. local time today, BP said this evening.

VIDEO: President Obama acknowledges the worst oil spill in U.S. history is his crisis.Play
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In a press conference at the White House this afternoon, President Obama took full responsibility for the government's response to the spill, defending his administration's decisions and calling the effort to contain and clean up the oil the largest of its kind in the nation's history.

"The American people should know that from the moment this disaster began, the federal government has been in charge of the response effort," he said. "BP is operating at our direction. Every key decision and action they take must be approved by us in advance."

Earlier in the day, the scientific team charged by Obama with determining the rate of oil flow estimated that the initial release following the accident was 12,000 to 19,000 barrels of oil per day, far higher than the 5,000 barrel-per-day estimate long maintained by BP and the U.S. Coast Guard.

"The original estimate was, of course, based on very limited data," said Marcia McNutt, the U.S. Geological Survey director who worked on the task force, which also included scientists from National Oceanic and Atmospheric Administration and Mineral Management Service.

The new spill estimate amounts to up to 798,000 gallons of oil per day since April 20, when the Deepwater Horizon rig blew up and later sank, larger than the volume of an entire Olympic-sized swimming pool.

Even under the most conservative estimate, nearly 17 million gallons would have leaked from the Deepwater Horizon well since the start of the disaster. On the high end of the estimate, some 29.5 million gallons may have been spilled. Either total would mean that the current crisis in the Gulf far surpasses the 1989 Exxon Valdez disaster, which spilled 11 million gallons of oil off the coast of Alaska.

Also today, Elizabeth Birnbaum, the head of the MMS, the government agency that regulates offshore drilling, resigned from her office under pressure.

'Top Kill': Looking for Signs of Progress

Unified Area Command and BP said today that they're still waiting for a sign as to whether the "top kill" operation they started Wednesday had been effective. BP believed that the technique, which pushes a mud-like drilling fluid into the well, has a 60 to 70 percent chance of success.

Earlier today, officials had said that the top kill had been working as planned, and that oil and gas had stopped flowing temporarily because of the mud injection.

"They are pumping mud into the well and as long as the mud is going down, the hydrocarbons are not coming up," said Coast Guard Adm. Thad Allen in an afternoon press conference. "The goal is to put enough mud into the well bore to produce zero pressure so they can put a cement plug over it. They are still in the process of doing that."

BP said it wouldn't be able to determine the success of the "top kill" for at least a day. If it fails, the company has other backup plans that include another cap and a so-called "junk shot" to clog the leak with debris.

MMS Director Steps Down Under Pressure

Former MMS Director Birnbaum was scheduled to testify at a hearing on Capitol Hill this morning, but shortly before the hearing started, her name plate was replaced.

"She has resigned," Interior Secretary Ken Salazar said during the hearing, calling her a "good public servant."

With the BP spill occurring on her watch, Birnbaum headed an agency that was sharply criticized by lawmakers and environmental groups, who argued that the agency had a cozy relationship with oil companies and failed to provide stringent regulation.

In a press conference this afternoon, President Obama would not say whether or not Birnbaum had been fired, saying that he had learned of the news this morning.

Last week, Obama announced that he was breaking up MMS into the three new agencies, reforming the way that oil drilling is regulated. Today, Obama cancelled some offshore drilling projects, pending further review of the BP spill.

Fishermen Become Ill While Working Near Oil

The same oil that has blighted the marshes and crippled the Gulf Coast economy is now apparently also threatening the health of some of the men who were trying to clean it up.

Nine fishermen became violently ill Wednesday while skimming oil in a nature reserve. Their symptoms included nausea, headaches and extreme dizziness. The men were taken to a hospital for treatment and examination.

Today, BP officials corralled 125 fishing boats and crews for advanced hazmat training, and officials are trying to determine the cause of the sickness.

"We're doing samples of the clothes they were wearing, we're looking at the food they took out there, and we are going to come to the best conclusion we can about what happened out there," Allen said this afternoon.

Some experts say that that chemicals in the crude oil and the dispersant used on it are evaporating, possibly causing the sickness, though the maker of the dispersant Corexit said that it is perfectly safe.

Still, the fishermen out there cleaning have no plan to stop.

"I have no choice [but to keep working for BP]," said A.C. Cooper, one of dozens of fishermen who said they've fallen ill from working around the oil. "Where do I go? Industry's shut down. Look around -- the boats are just sitting here. Can't work on an oil rig. What do you do?"

ABC's Jake Tapper, Dan Arnall, Ayana Harry and the Associated Press contributed to this report.