Crews prepared Thursday to lower a 100-ton box they hoped would cut off most of the crude spewing from a blown-out well in the Gulf of Mexico, the urgency of their task underscored by oil that started washing up on delicate barrier islands.

If the concrete-and-steel box they plan to lower a mile into the ocean works, it could collect as much as 85 percent of the oil that's been leaking from the ocean floor after the Deepwater Horizon drilling rig exploded April 20, killing 11 workers. The technique has not been tried before at that depth.

"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 with access to the containment effort.

It won't solve the problem altogether. Crews are drilling a relief well to take the pressure off the blown-out well at the site, and that could take up to three months.

More than 200,000 gallons of oil a day is pouring from the well, creating a massive sheen that's been floating on the Gulf for more than two weeks. As it moved closer to land, crews were frantically laying boom and taking other steps to prevent it from oozing into delicate coastal wetlands.

A pinkish oily substance was lapping at the shore of New Harbor Island, washing into thick marsh grass. It looked like soggy cornflakes, possibly because it was mixed with chemicals that it had been sprayed to break it up before it reached land.

Offshore, birds dove into the water amid lines of orange oil, but none appeared to be in distress. There were numerous dead jellyfish, some washing up on the shore. It's nesting time for sea gulls and pelicans and the danger is they may be taking contaminated food or oil on their feathers to their young.

People don't live on New Harbor, which is in the Chandeleurs, an important chain of barrier islands off Louisiana that are part of a national wildlife refuge and provide a nesting ground for sea birds.

At sea, some boats were using skimmers to suck up oil while others were corralling and setting fire to it to burn it off the surface.

The Joe Griffin, the ship carrying the containment box that will be lowered to the seafloor, arrived Thursday morning at the leak site about 50 miles offshore.

Workers hope to have the device down at the seabed by Thursday evening, but it will likely be Sunday or Monday before it's fully operational and they know if it's working.

The crew won't have to worry about dealing with the wreckage of the Deepwater Horizon, which sank two days after the explosion. It's not anywhere near where they're working. It had been operated by BP LPC, which is responsible for the cleanup.

The waters were calm Thursday with some clouds in the sky, though visibility was good. Roughly a dozen other ships either surrounded the site or could be seen in the distance. Thick, tar-like oil with a pungent scent surrounded the boat as far as the eye could see.

The semi-submersible drilling vessel Helix Q4000 was preparing to lift the box from the Joe Griffin with a crane and lower it to the seafloor. That was expected to happen later Thursday afternoon.

Oil has been leaking in three places since the explosion. One small leak was capped Wednesday. The containment box will be lowered over a much bigger leak in a pipe that's responsible for about 85 percent of the oil that's coming out.

The rest of the oil is coming from the blowout preventer at the well, a heavy piece of machinery designed to prevent blowouts that failed in the April 20 explosion. Crews have been trying to shut it off using robotic devices, but that hasn't worked.

If the box being lowered Thursday can contain the bigger leak, a second box being built may be used to stop the smaller leak at the blowout preventer.

The containment box has a dome-like structure at the top that's designed to act like a funnel and siphon the oil up through 5,000 feet of pipe and onto a tanker at the surface.

First, crews need to properly position the four-story structure with the help of a remote-controlled robotic submarine. A steel pipe will then be attached to a tanker at the surface and connected to the top of the dome to move the oil.

That process presents several challenges because of the frigid water temperature _ about 42 degrees Fahrenheit _ and exceptionally high pressure at those depths. Those conditions could cause the pipe to clog with what are known in the drilling industry as "ice plugs." To combat that problem, crews plan to continuously pump warm water and methanol down the pipe to dissolve the clogging.

They are also worried about the volatile cocktail of oil, gas and water when it arrives on the ship above. Engineers believe the liquids can be safely separated without an explosion.

"But of course we haven't done this before, it's very complex and we can't guarantee it," BP spokesman David Nicholas said Thursday.

Homeland Security Secretary Janet Napolitano said at a briefing in Biloxi, Miss., that officials were planning for the worst even though they hoped the device would work.

"If it does, of course, that will be a major positive development," she said.

BP engineers are also examining whether the leaking well could be shut off by plugging it from the top instead of drilling a relief well to cap it from the bottom.

The technique _ called a top kill _ would use a tube to shoot specialized mud and concrete directly into the top of the leaking 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.

No decision has been made on whether to use the technique.

The cause of the rig explosion is still not known, but investigators from multiple federal agencies are looking into the matter. A six-member investigative panel will begin its work next week.

The rig owner, Transocean Ltd., said in a filing with regulators Wednesday that it has received a request from the Justice Department to preserve information about the blast.

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Associated Press Writers Ray Henry in Robert, La., Brian Skoloff from Biloxi, Miss., and Chris Kahn in New York contributed to this report. Harry R. Weber reported from aboard the Joe Griffin supply boat carrying the containment box in the Gulf. Cain Burdeau reported from New Harbor Island, La.