JIM LEHRER: BP crews labored for a third day in the latest assault on that gushing oil well 50 miles off the Louisiana coast. Onshore, President Obama got to see the damage for himself.
Ray Suarez begins our coverage.
RAY SUAREZ: It was the president's second trip to the region since the spill began five weeks ago. While he was stepping off Air Force One in New Orleans, out in the Gulf, BP pumped more drilling fluid, called mud, into the blown wellhead overnight.
It sent brown clouds blossoming a mile below the surface. Coast Guard Admiral Thad Allen said there were signs that what's called a top kill procedure has had some success.
ADMIRAL THAD ALLEN, U.S. Coast Guard commandant: They have been able to push the hydrocarbons of the oil down with the mud. The real challenge is to put enough mud into the well to keep the pressure where they can put a cement plug over the top.
RAY SUAREZ: As robot subs labored on the seafloor, BP officials said the procedure was moving according to plan, despite an 18-hour delay yesterday.
CEO Tony Hayward urged patience.
TONY HAYWARD, CEO, BP Group: I think it's probably at least another 48 hours away before we could have confidence that we have succeeded. We have said all along that we rated this at 60 percent to 70 percent. That has not changed.
RAY SUAREZ: Hayward said engineers also fired debris into the wellhead at one point, including bits of shredded tires and golf balls. The idea was to give a new layer of mud something to stick to.
TONY HAYWARD: That's a combination of various -- materials of various sizes to attempt to create a bridging material. And, indeed, it is -- the non-technical term for it is junk, the junk shot.
RAY SUAREZ: But, later, there was word the operation had been halted again in the wee hours of the morning.
DOUG SUTTLES, COO, Global Exploration, BP: We have had periods of pumping followed by periods of monitoring. We have used our junk shot manifold and other loss-control material.
And I think, as we have stated before, we will continue with this operation until such time as it is either successful or we believe that it won't be successful.
RAY SUAREZ: Everyone, from BP, to federal officials, to Gulf Coast residents, hoped top kill and junk shot would become synonyms for success in stopping the worst oil spill in U.S. history. But, out on the water, and along the shoreline, the oil kept spreading, along with demands for tougher federal action.
Yesterday, at the White House, President Obama acknowledged the concern, and said he takes ultimate responsibility. Today, he stopped on Fourchon Beach in Louisiana, now closed because of the spill.
U.S. PRESIDENT BARACK OBAMA: You might want to stop right around here. You can see these little -- these little balls. These are the tar balls that they're talking about.
RAY SUAREZ: Later, the president received a formal briefing from Admiral Allen, then, flanked by Gulf Coast governors and other officials, told reporters in Grand Isle, Louisiana, he wanted to triple federal manpower in places where the oil is headed or has already made landfall.
BARACK OBAMA: This increase will allow us to further intensify this already historic response, contain and remove oil more quickly, and help minimize the time that any oil comes into contact with our coastline. That means deploying more boom, cleaning more beaches, performing more monitoring of wildlife and impact to this ecosystem.
RAY SUAREZ: Mr. Obama stressed again that BP is ultimately responsible for all cleanup costs. The oil company said the tab to date is $930 million and growing.