U.S. officials confirmed Thursday the first oil sheen to reach land from the massive spill in the Gulf of Mexico -- washing up on a beach on an uninhibited Louisiana island.
There had been reports for the past week that oil had washed ashore, but Coast Guard officials told FoxNews.com that until now, none of those reports had been confirmed by the government agencies responding to the spill.
The bulk of the oil spill remains off the coast, and what is arriving on shore now is "largely just sheen," Coast Guard Petty Officer Connie Terrell told Reuters. "There is no evidence of medium or heavy oil."
Officials were first notified Wednesday night about the oil on Freemason Island in the Chandeleur chain, and it was confirmed Thursday, a Coast Guard official told FoxNews.com.
Winds in the Gulf region are expected to pick up Saturday into Sunday causing the spill to be tougher to contain. Workers, meanwhile, gathered Thursday to begin lowering a giant concrete-and-steel box over the blown-out oil well at the bottom of the sea 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 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.
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.
FoxNews.com's Michelle Maskaly and The Associated Press contributed to this report.