(CNN) -- The estimated amount of oil spilling in an underwater leak from last week's oil rig explosion in the Gulf of Mexico has increased to 5,000 barrels a day, five times more than what was originally believed, a Coast Guard official said late Wednesday.

Rear Adm. Mary Landry said the increased estimate is based on analysis from the federal National Oceanic and Atmospheric Administration. She noted that there are "a lot of variables" in calculating the rate of the spill.

Additionally, a third underwater oil leak has been located in the pipeline that connected the rig to the oil well, said Doug Suttles, chief operating officer for BP, who joined Landry at a news conference. Two other leaks were located within 36 hours of the April 20 explosion.

The head of BP Group told CNN's Brian Todd in an exclusive interview Wednesday that the accident could have been prevented, and he focused blame on rig owner Transocean Ltd.

CEO Tony Hayward said Transocean's "blowout preventer" failed to operate before the explosion. A blowout preventer is a large valve at the top of a well, and activating it will stop the flow of oil. The valve may be closed during drilling if underground pressure drives up oil or natural gas, threatening the rig.

"That is the ultimate fail-safe mechanism," Hayward said. "And for whatever reason -- and we don't understand that yet, but we clearly will as a consequence of both our investigation and federal investigations -- it failed to operate.

"And that is the key issue here, the failure of the Transocean [blowout preventer]," Hayward said, describing the valve as "an integral part of the drilling rig," which is operated by Transocean.

A Transocean spokesman on Wednesday declined to respond to Hayward's comments, citing pending litigation against both companies. However, Transocean Vice President Adrian Rose has said its oil rig had no indication of problems before the explosion.

Asked whether the accident could have been prevented, Hayward said, "All accidents can be prevented -- there's not doubt about that."

BP owns the oil well. Before the explosion, Hayward had announced a significant discovery of at least 50 million barrels of oil. "Of course, all of that is completely irrelevant in the context of what we're now dealing with."

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What crews are dealing with is an oil spill from the explosion that continues to grow in the Gulf of Mexico. A controlled attempt to burn off part of the spill started Wednesday evening, the U.S. Coast Guard said.

Landry said the successful burn was completed later Wednesday night.

The burn-off is part of the effort to prevent the spread of oil from an underwater well that was broken open when the drill rig Deepwater Horizon blew up and sank last week. The slick stretched about 100 miles across the north-central Gulf on Wednesday afternoon and had advanced to within 16 miles of the mouth of the Mississippi River, the U.S. Coast Guard said.

Efforts to cap the well have so far been unsuccessful, Landry said Wednesday. The cause of the explosion remains under investigation, and search efforts have been halted for the 11 workers missing after the blast.

At least one of the victims' families has filed a lawsuit against BP and Transocean, accusing BP specifically of negligence.

"The responsibility for safety on the drilling rig is with Transocean," Hayward said. "It is their rig, their equipment, their people, their systems, their safety processes."

He said that, despite reports to the contrary, BP has not resisted attempts at tightening safety regulations. "We welcome tighter safety regulations. But we'd like them to be applied in a way that makes them practically impermeable."

BP and the Coast Guard planned to corral part of the oil slick using a 500-foot, specially designed boom, and then set it ablaze. The flames are expected to destroy between 50 to 90 percent of the oil in that section, and winds should blow the resulting cloud of smoke and soot out to sea, said Lt. Cmdr. Matt Moorlag, a Coast Guard spokesman.

"It's a historically proven technique, and it has multiple preventative safety measures in place to ensure that that burn area remains controlled," Moorlag said.

The oil spill has the potential to become one of the worst in U.S. history, Landry said Tuesday. The well, about 50 miles off the mouth of the Mississippi River, is dumping about 42,000 gallons a day into the Gulf of Mexico. BP, the well's owner, has been dropping dispersant chemicals on the slick and trying to shut off the flow using remote-controlled submarines, but has had no luck.

The spill stretched eastward from a point about 16 miles south of the mouth of the Mississippi River to about 94 miles south of Pensacola, Florida. It was about 30 miles wide at some points. Louisiana Gov. Bobby Jindal said the slick was expected to hit the southeastern Louisiana shoreline late Friday or before dawn Saturday.

Jindal said the state has asked for 55,000 feet of booms to keep oil away from the marshy, environmentally delicate coast, which is rich in shellfish and wildlife.

"We want to approach this situation the same way we would approach a hurricane or other natural disaster," he said. "We think it's best to hope for the best, but prepare for the worst."

The slick left many who draw their living from the water and coastal wetlands "watching and praying," said Tony Fernandez, owner of the Breton Sound Marina near Hopedale, Louisiana.

"For the most part, what we're doing is mostly waiting," Fernandez said. "There's not much that laypeople can do with this." People in the area are closely watching the Coast Guard plans to burn off portions of the slick, he said.

"I guess it's a question of water pollution or air pollution," he said.

Most of the slick is a thin sheen on the water's surface. About 3 percent of it is a heavy, pudding-like crude oil. At its current flow, the spill would take more than 260 days to rival the 1989 Exxon Valdez disaster, when about 11 million gallons spilled into Alaska's Prince William Sound.

But even if the Gulf spill never compares in size to the Exxon Valdez, it could have serious ecological repercussions if it makes landfall.

Efforts are already under way near the shoreline to deal with that potential scenario, including positioning boom material around sensitive ecological areas. Five staging areas have been set up on land, stretching from Venice, Louisiana, to Pensacola, Florida.

"If it reaches the shoreline, ourselves and the Coast Guard ... will deal with it," Hayward said. "And we will clean it up, if we get to that position."

He said BP's operation to attack the swath of oil, which is now the size of Delaware, is costing the company $6 million a day.

The remote-controlled submarines are being used by BP in an attempt to activate the well's blowout preventer to choke the flow of oil.

BP also is bringing in another drilling rig that could seal the well, but that effort would take months, according to a BP spokesman.

In the meantime, the company is working on a novel approach to capture the oil -- placing a dome right above the well head. The dome will resemble an inverted funnel, with a pipe leading up to ships waiting at the surface that would take away the oil. That tactic has never been tried in deep water, and a BP spokesman said the dome won't be ready for two to four weeks.