New Orleans ? A deepwater oil platform that burned for more than a day after a massive explosion sank into the Gulf of Mexico on Thursday, creating the potential for a major spill as it underscored the slim chances that the 11 workers still missing survived.

The sinking of the Deepwater Horizon, which burned violently until the gulf itself extinguished the fire, could unleash more than 300,000 gallons of crude a day into the water. The environmental hazards would be greatest if the spill were to reach the Louisiana coast, some 50 miles away.

Crews searched by air and water for the missing workers, hoping they had managed to reach a lifeboat, but one relative said family members have been told it’s unlikely any of the missing survived Tuesday night’s blast. The Coast Guard found two lifeboats but no one was inside. More than 100 workers escaped the explosion and fire; four were critically injured.

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

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

Jed Kersey, of Leesville, La., said his 33-year-old son, John, had finished his shift on the rig floor and was sleeping when the explosion occurred. He said his son told him that all 11 missing workers were on the rig floor at the time of the explosion.

“He said it was like a war zone,” said Jed Kersey, a former offshore oil worker.

An alarm sounded and the electricity went out, sending John Kersey and other workers scurrying to a lifeboat that took them to a nearby service boat, his father said.

“They waited for as many people as they could,” Jed Kersey said. He added that his son wasn’t ready to talk publicly about his experience.

As the rig burned, supply vessels shot water into it to try to keep it afloat and avoid an oil spill, but there were additional explosions Thursday. Officials had previously said the environmental damage appeared minimal, but new challenges have arisen now that the platform has sunk.

The well could be spilling up to 336,000 gallons of crude oil a day, Coast Guard Petty Officer Katherine McNamara said. She said she didn’t know whether the crude oil was spilling into the gulf. The rig also carried 700,000 gallons of diesel fuel, but that would likely evaporate if the fire didn’t consume it.

Coast Guard Rear Adm. Mary Landry said crews saw a 1-mile-by-5-mile rainbow sheen with a dark center of what appeared to be a crude oil mix on the surface of the water. She said there wasn’t any evidence crude oil was coming out after the rig sank, but officials also aren’t sure what’s going on underwater. They have dispatched a vessel to check.

The oil will do much less damage at sea than it would 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.

Doug Helton, incident operations coordinator for the National Oceanographic and Atmospheric Administration’s office of response and restoration, said the spill is not expected to come onshore in the next three to four days. “But if the winds were to change, it could come ashore more rapidly,” he said.

The well will need to be capped off underwater. Coast Guard Petty Officer Ashley Butler said crews were prepared for the platform to sink and had the equipment at the site to limit the environmental damage.