A starfish washes ashore Tuesday on the Chandeleur Islands, home of the Breton National Wildlife Refuge, off the southeastern coast of Louisiana. The barrier islands are at risk from a growing oil spill and leak in the Gulf of Mexico caused by the explosion and sinking of the Deepwater Horizon oil rig last week.

Biloxi, Miss. ? This time, it’s not a hurricane that threatens to wreck their livelihoods — it’s a blob of black ooze slowly making its way toward the Gulf Coast.

Hotel owners, fishermen and restaurateurs are keeping anxious watch as an oil slick spreads from a wrecked drilling rig site like a giant filthy ink blot. Forecasters say it could wash ashore within days near delicate wetlands, oyster beds and pristine white beaches.

Crews have not been able to stop thousands of barrels of oil from spewing out of the sea floor since an April 20 explosion destroyed the Deepwater Horizon, which was drilling 50 miles off the Louisiana coast. Eleven workers are missing and presumed dead, and the cause of the explosion has not been determined.

Louis Skrmetta, 54, runs a company called Ship Island Excursions that takes tourists to the Gulf Islands National Seashore, where white-sand beaches and green water create an idyllic landscape.

“This is the worst possible thing that could happen to the Mississippi Gulf Coast,” he said. “It will wipe out the oyster industry. Shrimping wouldn’t recover for years. It would kill family tourism. That’s our livelihood.”

As crews struggled to contain the oil slick, Coast Guard officials said Tuesday they were considering setting fire to the contaminated water to burn off the crude. Pools of oil far offshore would be trapped in special containment booms and set aflame as soon as today.

“If we don’t secure this well, this could be one of the most significant oil spills in U.S. history,” Coast Guard Rear Adm. Mary Landry said.

A similar burn off the coast of Newfoundland in 1993 eliminated 50 percent to 99 percent of captured oil. However, burning the oil also creates air pollution, and the effect on marine life is unclear.

Ed Overton, a professor emeritus of environmental sciences at Louisiana State University who’s studying the oil spill, questioned whether burning would work.

“It can be effective in calm water, not much wind, in a protected area,” he said. “When you’re out in the middle of the ocean, with wave actions, and currents, pushing you around, it’s not easy.”

He has another concern: The oil samples from the spill he’s looked at shows it to be a sticky substance similar to roofing tar.

“I’m not super optimistic. This is tarry crude that lies down in the water,” he said. “But it’s something that has got to be tried.”

According to the National Oceanic and Atmospheric Administration, birds and mammals are more likely to escape a burning area of the ocean than escape from an oil slick. The agency said birds might be disoriented by the plumes of smoke, but they would be at much greater risk from exposure to oil in the water.

In Washington, meanwhile, the Obama administration launched a full investigation of the explosion, and Homeland Security Secretary Janet Napolitano and Interior Secretary Ken Salazar said they will devote every available resource to the probe.

Meanwhile, the glistening sheen of sweet crude continued to grow and began forming long reddish-orange ribbons of oil that, if they wash up on shore, could cover birds, white sand beaches and marsh grasses.

The last major spill in the Gulf was in June 1979, when an offshore drilling rig in Mexican waters — the Ixtoc I — blew up, releasing 140 million gallons of oil. It took until March 1980 to cap the well, and a great deal of the oil contaminated U.S. waters and Texas shores.

“In the worst-case scenario, this could also last months,” said Richard Haut, a senior research scientist at the Houston Advanced Research Center who worked for Exxon for 20 years, 10 of them on an offshore platform in the North Sea.

Oil from the Deepwater Horizon is not expected to reach the coast until late in the week, if at all. As of Tuesday, it was about 20 miles offshore, south of Venice, La. The spill covered an expanding area about 48 miles long and 80 miles wide, but with uneven borders, making it difficult to calculate its area in square miles.