The British oil giant said it was ahead of its schedule to get the new cap in place. Once the cap is in place and working properly, officials hoped it would capture all of the oil spewing out of the well and that it can all be funneled to containment ships at the surface.

"The hope is that we can slowly turn off the valves, close the capping completely and then test pressure to see how the well is performing," Thad Allen, the government's point man on the disaster, said on CBS television.

While the operation is under way, the previous cap had to be removed — meaning all of the oil is escaping unfettered until the new cap can be installed. Still, the chance to capture all the oil was a welcome bit of news 83 days into the environmental and economic disaster that has fouled the Gulf and its fragile coastline.

After more than two months of failed efforts, there remains a healthy dose of skepticism among those who live and work along the coast.

"At this point, there have been so many ups and downs, disappointments, that everybody down here is like, 'We'll believe it when we see it,"' said Keith Kennedy, a charter boat captain in Venice, Louisiana.

In a regulatory filing Monday, BP PLC said the installation of the sealing cap was proceeding as planned.

A transition spool had been installed on the existing flange. The next step was to install a capping stack that has three closing rams.

It was unclear from undersea video feeds and the comments in the filing if the process of lowering the new cap had begun early Monday. Several spokesman did not respond to e-mails and phone calls seeking comment early Monday, and people who answered phone calls to vessels involved in the containment effort declined to comment.

Also Monday, BP said in the Securities and Exchange Commission filing that the cost of the response to date has risen to roughly $3.5 billion. That includes the cost of the spill response, containment, relief well drilling, grants to the Gulf states, claims paid and federal costs.

Once the new cap was placed atop the well it was expected to provide a tight seal that should eventually allow the oil giant to capture all of the leaking crude for the first time since the April 20 oil rig explosion set off the environmental crisis. But prior failed attempts to stop the leak have made BP careful to keep expectations grounded.

BP has tried and failed to counter the leak with a giant concrete box over the well, mud and shredded rubber pumped into it and a pipe to siphon the crude. A converted supertanker specially equipped to skim huge amounts of oil from the surface has been hampered by bad weather.

Gulf residents and politicians reserved judgment about BP's latest effort and said damage already done to the environment, fishing and tourism will haunt the region.

"I'm not a scientist, but I know a lot of people are praying that they get that flow stopped," said Florida Governor Charlie Crist, who was attending a Jimmy Buffet beach benefit concert Sunday in Alabama.

Roughly 81,000 square miles (210,000 square kilometers) of federal waters in the Gulf have been closed to fishing since the beginning of the disaster, about 44 percent of the total.

The well has been gushing largely unchecked since an old, leaky cap was removed from the wellhead Saturday afternoon to make way for the new one.

BP senior vice president Kent Wells said Sunday afternoon he's pleased with the progress, but hastened to add the operation was still expected to last another two to five more days.

Officials won't be satisfied the cap is working until they've run tests on whether it can withstand the tremendous pressure of oil pushing up from below the seafloor.

Asked during a conference call if the new cap and collection efforts would end the spilling of oil into the Gulf, Wells said only that BP will capture all the oil "at some point." The new cap will be aided in containing the leak by the arrival of the Helix Producer, a vessel that should be able to take in about 1 million gallons (3.8 million liters) of crude per day after coming online. The Helix connected to flexible pipes from the well Friday, and crews have been running tests since then.

Ultimately, the plan is to have four vessels collecting oil from the leak with a combined capacity of about 2.5 million gallons to 3.4 million gallons (9.5 million liters to 13 million liters) a day — enough to capture all the oil leaking, if federal estimates are right. Getting all the vessels on the task will take about two to three weeks.

The new, tighter cap is not intended to be the permanent solution to the problem.

Relief wells are being dug for the final fix, a "bottom kill" in which heavy drilling mud and cement are pumped in from below the broken wellhead.

BP and government officials have said the relief wells are expected to be completed sometime around mid-August.