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The U.S. Coast Guard has called off its search for 11 workers who have been missing from an oil rig that exploded and sank in the Gulf of Mexico off the Louisiana coast this week.

Rear Adm. Mary Landry described the decision to call off the search a very difficult one.

Rescue crews have spent three days searching a large area surrounding the rig, but could not find the missing workers. Coast guard officials searched by air and sea, hoping the workers had made it to a lifeboat after the explosion.

The coast guard said it will resume the search if any ships in the area see anything.

Earlier, rescue crews found two lifeboats — but no one was inside the boats. More than 100 workers escaped the Tuesday explosion and fire, while four were critically injured.

Carolyn Kemp of Monterey, La., said her grandson, Roy Wyatt Kemp, 27, is among the missing. She said he would have been on the drilling platform when the explosion occurred.

"They're assuming all those men who were on the platform are dead," Kemp said. "That's the last we've heard."

Adrian Rose, vice-president of rig owner Transocean Ltd., said Thursday some surviving workers told the company their missing colleagues may not have been able to flee the rig in time. He said he was unable to confirm whether that was the case.

Meanwhile, no oil appears to be leaking from the drilling rig, the coast guard said.

The explosion ripped through the Deepwater Horizon rig Tuesday, sparking a major fire that burned until Thursday, when the rig collapsed and sank.

By The Numbers

by-the-numbers look  at the sunken oil rig and the cleanup effort.

Oil giant BP, which contracted the rig, said it has mobilized four aircraft that can spread chemicals to break up oil and 32 vessels — including a big storage barge — that can suck more than 171,000 barrels of oil a day from the surface.

Mike O'Berry, a senior chief petty officer with the coast guard, said efforts are underway to contain oil that spilled into the water after the explosion. He said BP is the "responsible party" and will lead cleanup of oil that spilled during the fire, but he said the coast guard will oversee the effort.

"It's a very collaborative effort," O'Berry said. "It takes everybody to make this happen."

A large oil slick has developed on the water's surface, but O'Berry said underwater robots have shown there are no underwater leaks.

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In this aerial photo taken in the Gulf of Mexico, the Deepwater Horizon oil rig is seen burning Wednesday. ((Gerald Herbert/Associated Press))

The rig is roughly 80 kilometres off the Louisiana coast. If more oil does leak from the rig, it will do much less damage at sea than if it hits the shore, said Cynthia Sarthou, executive director of the Gulf Restoration Network.

"If it gets landward, it could be a disaster in the making," Sarthou said.

Since 2001, there have been 69 offshore deaths, 1,349 injuries and 858 fires and explosions in the gulf, according to the Minerals Management Service. O'Berry said accidents are rare, given that 30,000 people work on rigs there every day.

"They're highly trained. They know the dangers," O'Berry said. "The safety precautions they take are extreme. A testament to that is, of the 126 [workers], 115 are home today with their families."

With files from The Associated Press