VENICE, Louisiana -- BP's chairman defended his company's safety record and said Sunday that "a failed piece of equipment" was to blame for a massive oil spill along the Gulf Coast, where President Barack Obama was headed for a firsthand update on the slick creeping toward American shores.

BP PLC chairman Lamar McKay told ABC's "This Week" that he can't say when the well a mile (1.6 kilometers) beneath the sea might be plugged. But he said he believes a dome that could be placed over the well is expected to be deployed in six to eight days.

The dome has been made and workers are finishing the plan to get it deployed, McKay said. He said BP officials are still working to activate a "blowout preventer" mechanism meant to seal off the geyser of oil.

"And as you can imagine, this is like doing open-heart surgery at 5,000 feet (1,500 meters), with -- in the dark, with robot-controlled submarines," McKay said.

BP spokesman Bill Salvin said McKay was talking about the blowout preventer as the failed equipment that caused the April 20 explosion of the Deepwater Horizon rig, which killed 11 people. The blowout preventer typically activates after a blast or other event to cut off any oil that may spill.

The cause of the blast remains undetermined, and Salvin said "we're not ruling anything out."

Crews have had little success stemming the flow from the ruptured well on the sea floor off Louisiana or removing oil from the surface by skimming it, burning it or dispersing it with chemicals. The churning slick of dense, rust-colored oil is now roughly the size of Puerto Rico.

Adding to the gloomy outlook were warnings from experts that an uncontrolled gusher could create a nightmare scenario if the Gulf Stream current carries it toward the Atlantic.

Long tendrils of oil sheen made their way into South Pass, a major channel through the salt marshes of Louisiana's southeastern bootheel that is a breeding ground for crab, oysters, shrimp, redfish and other seafood.

Venice charter boat captain Bob Kenney lamented that there was no boom in the water to corral the oil, and said BP was "pretty much over their head in the deep water."

"If they weren't, they would have cut the oil off by now," he said.

"It's like a slow version of Katrina," he added. "My kids will be talking about the effect of this when they're my age."

About a half-dozen fishing vessels sailed Sunday morning through the marshes of coastal St. Bernard Parish in eastern Louisiana, headed for the Biloxi Wildlife Management area. The oyster and shrimp boats, laden with boom, hoped to seal off inlets, bayous and bays.

There is growing criticism that the government and oil company BP PLC should have done more to stave off the disaster, which cast a pall over the region's economy and fragile environment. Moving to blunt criticism that the Obama administration has been slow in reacting to the largest U.S. crude oil spill in decades, the White House dispatched two Cabinet members to make the rounds on the Sunday television talk shows.

Homeland Security Secretary Janet Napolitano said on "Fox News Sunday" that the government has taken an "all hands on deck" approach to the spill since the BP oil well ruptured.

Napolitano said that as BP officials realized more oil was spewing than first thought, the government has coordinated federal, state and local resources with the oil company's response.

Interior Secretary Ken Salazar told NBC's "Meet the Press" that it could take three months before workers attain what he calls the "ultimate solution" to stopping the leak -- drilling a relief well more than 3 miles (5 kilometers) below the ocean floor.

However, as the spill surged toward disastrous proportions, critical questions lingered: Who created the conditions that caused the gusher? Did BP and the government react robustly enough in its early days? And, most important, how can it be stopped before the damage gets worse?

The Coast Guard and BP have said it's nearly impossible to know exactly how much oil has gushed since the blast, though it has been roughly estimated the well was spewing at least 200,000 gallons (757,000 liters) a day.

Even at that rate, the spill should eclipse the 1989 Exxon Valdez incident as the worst U.S. oil disaster in history in a matter of weeks. But a growing number of experts warned that the situation may already be much worse.

The oil slick over the water's surface appeared to triple in size over the past two days, which could indicate an increase in the rate oil is pouring from the well, according to one analysis of images collected from satellites and reviewed by the University of Miami. While it's hard to judge the volume of oil by satellite because of depth, images do indicate growth, experts said.

"The spill and the spreading is getting so much faster and expanding much quicker than they estimated," said Hans Graber, executive director of the university's Center for Southeastern Tropical Advanced Remote Sensing.

In an exploration plan and environmental impact analysis filed with the federal government in February 2009, BP said it had the capability to handle a "worst-case scenario" at the site, which the document described as a leak of 162,000 barrels per day from an uncontrolled blowout -- 6.8 million gallons (26 million liters) each day.

Oil industry experts and officials are reluctant to describe what, exactly, a worst-case scenario would look like. But if the oil gets into the Gulf Stream and carries it to the beaches of Florida -- and potentially loops around the state's southern tip and up the eastern seaboard -- several experts said it stands to be an environmental and economic disaster of epic proportions.

"It will be on the East Coast of Florida in almost no time," Graber said. "I don't think we can prevent that. It's more of a question of when rather than if."

The concerns are both environmental and economic. The fishing industry is worried marine life will die -- and that no one will want to buy products from contaminated water anyway. Tourism officials are worried vacationers won't want to visit oil-tainted beaches. And environmentalists are worried about how the oil will affect the countless birds, coral and mammals in and near the Gulf.

"We know they are out there," said Meghan Calhoun, a spokeswoman from the Audubon Aquarium of the Americas in New Orleans. "Unfortunately, the weather has been too bad for the Coast Guard and NOAA to get out there and look for animals for us."

Fishermen and boaters want to help but have been hampered by high winds and rough waves that render oil-catching booms largely ineffective. Some coastal Louisiana residents complained that BP was hampering mitigation efforts.

"No, I'm not happy with the protection, but I'm sure the oil company is saving money," said 57-year-old Raymond Schmitt, in Venice preparing his boat to take a French television crew on a tour.

And the oil on the surface is just part of the problem. Louisiana State University professor Ed Overton, who heads a federal chemical hazard assessment team for oil spills, worries about a total collapse of the pipe inserted into the well. If that happens, there would be no warning and the resulting gusher could be even more devastating.

"When these things go, they go KABOOM," he said. "If this thing does collapse, we've got a big, big blow."

BP has not said how much oil is beneath the seabed Deepwater Horizon was tapping. A company official, speaking on condition of anonymity because he was not authorized to discuss the volume of reserves, confirmed reports that it was tens of millions of barrels.

Obama has halted any new offshore drilling projects unless rigs have new safeguards to prevent another disaster.

As if to cut off mounting criticism, on Saturday White House spokesman Robert Gibbs posted a blog entitled "The Response to the Oil Spill," laying out the administration's day-by-day response since the explosion, using words like "immediately" and "quickly," and emphasizing that Obama "early on" directed responding agencies to devote every resource to the incident and determining its cause.