ON THE GULF OF MEXICO (AP) — Workers gathered to begin lowering a giant concrete-and-steel box over the blown-out oil well at the bottom of the sea Thursday in a risky and untested bid to capture most of the gushing crude and avert a wider environmental disaster.

‘‘We haven’t done this before. It’s very complex and we can’t guarantee it,’’ BP spokesman David Nicholas warned.

The 100-ton containment vessel is designed to collect as much as 85 percent of the oil spewing into the Gulf and funnel it up to a tanker. It could take several hours to lower it into place by crane, after which a steel pipe will be installed between the top of the box and the tanker. The whole structure could be operating by Sunday.

The technology has been used a few times in shallow waters, but never at such extreme depths — 5,000 feet down, where the water pressure is enough to crush a submarine.

The box — which looks a lot like a peaked, 40-foot-high outhouse, especially on the inside, with its rough timber framing — must be accurately positioned over the well, or it could damage the leaking pipe and make the problem worse.

Other risks include ice clogs in the pipes — a problem that crews will try to prevent by continuously pumping in warm water and methanol — and the danger of explosion when separating the mix of oil, gas and water that is brought to the surface.

‘‘I’m worried about every part, as you can imagine,’’ said David Clarkson, BP vice president of engineering projects.

If the box works, a second one now being built may be used to deal with a second, smaller leak from the sea floor.

‘‘Hopefully, it will work better than they expect,’’ first mate Douglas Peake told The Associated Press aboard the ship that brought the box to the site. The AP is the only news organization on board the vessel.

The well blew open on April 20 when the Deepwater Horizon drilling platform exploded 50 miles out in the Gulf of Mexico, killing 11 workers. The well has been spewing an estimated 200,000 gallons a day in the nation’s biggest oil spill since the Exxon Valdez disaster in Alaska in 1989.

Interior Secretary Ken Salazar on Thursday halted all new offshore drilling permits nationwide until at least the end of the month while the government investigates the Gulf spill.

Oil slicks stretched for miles off the Louisiana coast, where desperate efforts were under way to skim, corral and set the petroleum ablaze. People in Mississippi, Alabama and Florida watched in despair.

The dropping of the box is just one of many strategies being pursued to stave off a widespread environmental disaster. BP is drilling sideways into the blown-out well in hopes of plugging it from the bottom. Also, oil company engineers are examining whether the leak could be shut off by sealing it from the top instead.

The technique, called a ‘‘top kill,’’ would use a tube to shoot mud and concrete directly into the well’s blowout preventer, BP spokesman Bill Salvin said. The process would take two to three weeks, compared with the two to three months needed to drill a relief well.

On Thursday, oil reached several barrier islands off the Louisiana coast, many of them fragile animal habitats. Several birds were spotted diving into the oily, pinkish-brown water, and dead jellyfish washed up on the uninhabited islands.

‘‘It’s all over the place. We hope to get it cleaned up before it moves up the west side of the river,’’ said Dustin Chauvin, a 20-year-old shrimp boat captain from Terrebonne Parish, La. ‘‘That’s our whole fishing ground. That’s our livelihood.’’

During a visit to Biloxi, Miss., Homeland Security Secretary Janet Napolitano said of the containment vessel: ‘‘I hope it works. But we are still proceeding as if it won’t. If it does, of course, that will be a major positive development.’’

‘‘We are facing an evolving situation,’’ she warned. ‘‘The possibility remains that the BP oil spill could turn into an unprecedented environmental disaster. The possibility remains that it will be somewhat less.’’

Meanwhile, a six-member board composed of representatives of the Coast Guard and the federal Minerals Management Service will begin investigating the accident next week.

And a federal judicial panel in Washington has been asked to consolidate at least 65 potential class-action lawsuits claiming economic damage from the spill. Commercial fishermen, business and resort owners, charter boat captains, even would-be vacationers have sued from Texas to Florida, seeking damages that could reach into the billions.

‘‘It’s just going to kill us. It’s going to destroy us,’’ said Dodie Vegas, who owns a motel and cabins in Grand Isle, La., and has seen 10 guests cancel.