VENICE, La. (AP) — Federal officials shut down fishing from the Mississippi River to the Florida Panhandle on Sunday because of the uncontrolled gusher spewing massive amounts of oil into the Gulf of Mexico, and the environmental disaster is still expected to take at least a week to cut off.

Even that toxic scenario may be too rosy because it depends on a low-tech strategy that has never been attempted before in deep water.

The plan: to lower 74-ton, concrete-and-metal boxes into the gulf to capture the oil and siphon it to a barge waiting at the surface. Whether that will work for a leak 5,000 feet below the surface is anyone’s guess; the method has previously worked only in shallower waters.

If it doesn’t, and efforts to activate a shutoff mechanism called a blowout preventer continue to prove fruitless, the oil probably will keep gushing for months until a second well can be dug to cut off the first. Oil giant BP PLC’s latest plan will take six to eight days because welders have to assemble the boxes.

President Obama toured the region Sunday, deflecting criticism that his administration was too slow to respond and did too little to stave off the catastrophe.

Satellite images indicate the rust-hued slick tripled in size in just two days, suggesting the oil could be pouring out faster than before. Wildlife including sea turtles have been found dead on the shore but it is too soon whether the spill, caused by an April 20 oil rig explosion, was to blame.

Even if the well is shut off in a week, fishermen and wildlife officials wonder how long it will take for the gulf to recover. Some compare it to the hurricane Louisiana is still recovering from after nearly five years.

‘‘It’s like a slow version of Katrina,’’ Venice charter boat captain Bob Kenney said. ‘‘My kids will be talking about the effect of this when they’re my age.’’

More than 6,800 square miles of federal fishing areas, from the mouth of the Mississippi to Florida’s Pensacola Bay were closed for at least 10 days on Sunday by the National Oceanic and Atmospheric Administration. NOAA Administrator Jane Lubchenco says government scientists are taking samples from the waters near the spill to determine whether there is any danger.

Fishermen still were out working, however: They have been dropping miles of inflatable, oil-capturing boom around the region’s fragile wetlands and prime fishing areas. Bad weather, however, was thwarting much of the work; Alabama Gov. Bob Riley said 80 percent of the booms laid down off his state over the previous three days had broken down. He said booms along other coasts is breaking down also.

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 to be at least 200,000 gallons a day.

At that rate, it would eclipse the 1989 Exxon Valdez tanker spill — which dumped 11 million gallons off the Alaska coast — as the worst U.S. oil disaster in history in a matter of weeks.

‘‘None of us have ever had experience at this level before. It ain’t good,’’ said Bob Love, coastal and nongame resources administrator with the Louisiana Department of Wildlife and Fisheries.

‘‘The longer it goes, the more fish and wildlife impacts there will be.’’

Even if the oil stays mostly offshore, the consequences could be dire for sea turtles, dolphins and other deepwater marine life — and microscopic plankton and tiny creatures that are a staple of larger animals’ diets.

Moby Solangi, director of the Institute for Marine Mammal Studies in Gulfport, Miss., said at least 20 dead sea turtles were found on the state’s beaches. He said it’s too soon to say whether oil contamination killed them but that it is unusual to have them turning up across such a wide stretch of coast, spanning nearly 30 miles.

None of the turtles have oil on them, but Solangi said they could have ingested oily fish or breathed in oil on the surface. Necropsies will be performed Monday.

The situation could become even more grave if the oil gets into the Gulf Stream and flows to the beaches of Florida — and potentially whips around the state’s southern tip and up the Eastern Seaboard. Tourist-magnet beaches and countless wildlife could be ruined.

Obama has halted any new offshore drilling projects unless rigs have new safeguards to prevent another disaster. On Sunday he called the spill a ‘‘massive and potentially unprecedented environmental disaster,’’ and made clear that he was not accepting blame.

‘‘BP is responsible for this leak. BP will be paying the bill,’’ he said, rain dripping from his face in Venice, a Gulf Coast community serving as a staging area for the response.

Homeland Security Secretary Janet Napolitano has said any comparison between the ruptured BP oil well and Katrina was ‘‘a total mischaracterization’’ and that the government has taken an ‘‘all hands on deck’’ approach.

Plaquemines Parish president Billy Nungesser met with Obama Sunday in Venice and said the president’s ‘‘let’s-get-it-done approach was overwhelming.’’ He said he hopes that the president pressures BP to approve funding for a plan to have fishermen start laying oil-sopping booms very soon.

Obama acknowledged the booms haven’t been working well out in the choppy gulf, Nungesser said.

After the oil rig explosion, which killed 11 people, the flow of oil should have been stopped by a blowout preventer, but the mechanism failed, as have continuing efforts to activate it.

‘‘As you can imagine, this is like doing open-heart surgery at 5,000 feet, with — in the dark, with robot-controlled submarines,’’ BP PLC Chairman Lamar McKay said on ABC’s ‘‘This Week.’’ He defended his company’s safety record.

Charlie Holt, drilling and completing operations manager for BP in the Gulf of Mexico, said Sunday it’s still unclear why the blowout preventer failed. But he suggested something could be in the way, or it could be damaged.

‘‘There is nothing unique about the situation that should have prohibited the BOP from working as designed,’’ Holt said of the device.

The containment boxes being built to stop the leak — 40 feet tall, 24 feet wide and 14 feet deep — were not part of the company’s original response plan. But they appear to be the best hope for keeping the oil well from gushing for months.

The approach has been used previously only for spills in relatively shallow water. Coast Guard Adm. Thad Allen said engineers are still examining whether the valves and other systems that feed oil to a ship on the surface can withstand the extra pressures of the deep.

‘‘This is a completely new way of dealing with this problem,’’ said Greg Pollock, commissioner of the oil spill prevention and response program at the Texas General Land Office. ‘‘Generally speaking, nobody’s ever tried anything like this on this scale.’’

If the boxes don’t work, BP also has begun work on its only other backup plan: two relief wells that will take as long as three months to drill.

Bob Fryar, senior vice president for BP in Angola, said the Bureau of Minerals Management has approved a plan to drill the wells. BP wants to drill two in case one doesn’t work.

He said teams are working 24 hours a day on every possible solution. He said 389 people reported to an emergency center at BP’s Houston office alone.

‘‘What BP’s doing is is throwing absolutely everything we can at this,’’ he said Sunday. ‘‘We ceratainly want to do everything we can, everything we can possibly think of, as a company, as an industry.’’

BP has not said how much oil is beneath the seabed the Deepwater Horizon rig was tapping when it exploded. 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. Fryar said any numbers being thrown out are just estimates at best. The rig was operated by BP and owned by Transocean Ltd.

Teams working to contain the spill have had limited success using airplanes to drop chemical dispersants meant to break up the oil, and rough seas have prevented ships from skimming crude from the surface.

In Alabama, National Guard soldiers arrived on the state’s Dauphin Island to build a berm meant to buffer part of the island against the slick.

However, the oil on the surface is just part of the problem. There could be far more oil unseen below the surface. And the pipe sprouting from the well could completely collapse — leading to an even more devastating gusher.

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Associated Press writers Jay Reeves, Mike Graczyk, Tamara Lush, Brian Skoloff, Melissa Nelson, Mary Foster, Chris Kahn, Vicki Smith, John Flesher, Holbrook Mohr and AP Photographer Dave Martin contributed to this report.