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.
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.
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.