Last Updated 6:38 p.m. ET
The Coast Guard is calling off its search for 11 workers missing since an explosion on an oil rig off the Louisiana coast.
Rear Adm. Mary Landry calls it a very difficult decision.
The Coast Guard said Friday crews had spent three days searching a large area surrounding the rig, but could not find the missing 11. Coast Guard officials searched by air and sea, hoping the workers had made it to a lifeboat after the explosion.
Transocean Ltd. owns the platform. Nearly 100 made it off the Deepwater Horizon safely after Tuesday night's blast. Seventeen others were hurt.
The Coast Guard says it will resume the search if any ships in the area see anything.
Earlier Friday, authorities were given good news about the potential for a massive oil spill that could threaten the fragile ecosystem of the Louisiana and Mississippi coasts.
Landry, the U.S. Coast Guard commander for the region, told CBS' "Early Show" co-anchor Harry Smith Friday morning that, according to underwater monitoring equipment in place, "no crude is emanating from that wellhead."
She said no gas or oil appeared to be hemorrhaging from the sunken well's massive reserves, either.
CBS News Correspondent Don Teague reports there is a massive cleanup effort underway. A half dozen oil recovery ships have dropped nearly 100 miles of boom to contain 200 barrels of oil spread over 16 square miles, enough to threaten wildlife if it reaches shore.
Landry said there was a "one mile by about a 12-mile rainbow sheen" in the Gulf with "pockets of crude" left behind by significant amounts of fuel oil that burned off immediately following the devastating explosion.
The Deepwater Horizon had burned violently for nearly two days until it sank Thursday morning. The fire's out, but as much as 336,000 gallons of crude oil a day could start rising from the sea floor 5,000 feet below, officials fear.
"If it gets landward, it could be a disaster in the making," said Cynthia Sarthou, executive director for the environmental group Gulf Restoration Network.
"There are important resources on the coast. Our Fish and Wildlife Service has refuges, the state has assets, obviously there are important communities. All that is being taken into account," said David Jay Hayes, Deputy Secretary of the Department of the Interior.
Landry said the Coast Guard was continuously monitoring the well and the rig, from both the surface and the seafloor, to watch for any leakage.
BP PLC, which leased the rig and took the lead in the cleanup, said Friday it has "activated an extensive oil spill response," including using remotely operated vehicles to assess the subsea well and 32 vessels to mop up the spill.
BP Chief Executive Tony Hayward said the company will do "everything in our power to contain this oil spill and resolve the situation as rapidly, safely and effectively as possible." He says the company can call on more resources if needed.
Ed Overton, an LSU environmental sciences professor, said he expects some of the light crude oil to evaporate while much of it turns into a pasty mess called a "chocolate mousse" that ultimately breaks apart into "tar balls," small chunks of oily residue that can wash ashore.
"It's going to be a god-awful mess for a while," he said. "I'm not crying doomsday or saying the sky is falling, but that is the potential."
The Coast Guard early Friday was searching for the missing, but some family members said they had been told that officials assumed all were dead, though others were praying for a different outcome. Most of the crew - 111 members - were ashore, including 17 taken to hospitals. Four were in critical condition.
"During the debriefing of the survivors, it was clarified that the 11 missing members may have been in the vicinity of the explosion," said Landry.
She told CBS News there was still an "active search and rescue" operation underway, but CBS News correspondent Whit Johnson reports that effort was scaled back overnight.
The accident shows that drilling is not safe, said Abe Powell, who heads Get Oil Out!, created after a 1969 platform accident off Santa Barbara, Calif., fouled miles of ocean and beaches with wildlife-killing goo and spawned the environmental movement.
"When oil companies say drilling is safe now and we won't allow any accidents ... we know that's not true," he said.
Meanwhile, U.S. Sen. Bill Nelson called for a congressional investigation of safety practices at offshore oil rigs. Nelson, a Democrat who has led opposition to offshore drilling, said he asked the U.S. Interior Department to investigate and provide a comprehensive report on all U.S. drilling accidents over at least the last decade.
"The tragedy off the coast of Louisiana shows we need to be asking a lot more tough questions of big oil," Nelson said. "I think we need to look back over 10 years or so to see if the record denies the industry's claims about safety and technology."
The Coast Guard, which was leading the investigation, hadn't given up the search early Friday for those missing from the rig, which went up in flames Tuesday night about 41 miles from the mouth of the Mississippi River.
Lt. Cmdr. Cheri Ben-Iesau said a Coast Guard cutter would remain on the scene Friday after searching overnight, and a helicopter would take advantage of clear weather to make three more search flights.
"We use a scientific program to make a best-guess estimate on survivability," Ben-Iesau said. "And then the Coast Guard searches a little longer than that. Because there's always the unknown."
Four who made it off safely were still on a boat operating one of several underwater robots being used to assess whether the flow of oil could be shut off at a control valve on the sea floor, said Guy Cantwell, spokesman for rig owner Transocean Ltd.
Weather forecasts indicate the spill was likely to stay well away from shore at least through the weekend, but if winds change it could come ashore more rapidly, said Doug Helton of the National Oceanographic and Atmospheric Administration's office of response and restoration.
At the worst-case figure of 336,000 gallons a day, it would take more than a month for the amount of crude oil spilled to equal the 11 million gallons spilled from the Exxon Valdez in Alaska's Prince William Sound.
A turn in winds and currents might send oil toward fragile coastal wetlands - nurseries for fish and shrimp and habitat for birds.
"As you get closer to shore, you get richer and richer marine habitats, and also get the potential for long-term exposure," Helton said.
Helton, of NOAA, said it is probably good that the sheen is spreading because it will be exposed to more waves and sunlight that will help break it down. But he said a wider area also means a greater chance that seabirds and marine mammals will be affected.
Overton said the sheen's distance from shore means the impact on wildlife is likely not widespread, although some seabirds that dive for food could become coated with oil.
The problem, Overton said, would be if thicker globs of oil reach coastal areas such as the Chandeleur islands, home to hatcheries for pelicans and other birds.
Animals at sea will be briefly exposed to the oil when the slick passes over, but when it hits land, it sticks, he said.
To prevent that, the Marine Spill Response Corp., an energy industry cleanup consortium, brought seven skimmer boats to suck oily water from the surface, four planes that can scatter chemicals to disperse oil, and 500,000 feet - 94.6 miles - of containment boom.
Another 500,000 feet of boom were on the way, BP spokesman Tom Mueller.
"Right now we are over-responding with resources to manage the potential spill here," he said. "We will be well-prepared to manage whatever comes."
He said 6,000 feet, about 1.1 mile, of boom was in the water by Thursday evening.
While this was happening on the surface, robots tethered to ships nearly a mile above the sea floor sent back video of the damage so crews can decide whether a shutoff valve called a blowout preventer can be closed.
Authorities don't know whether the rig sank to the bottom - or, if it did, whether it hit the blowout preventer, Lt. Cdr. Cheri Ben-Iesau said.
"It didn't sink catastrophically. It kind of settled into the water" and may still have some buoyancy, she said.
If the valve is too badly damaged to cut off the flow of oil, a nearby rig a safe distance from the broken well will drill a new hole intersecting the one that blew wild. Then heavy fluid called "kill fluid" will be pumped in to plug it, said Scott D. Dean, a BP spokesman.
In addition to other environmental concerns, the well is in an area where a pod of sperm whales is known to feed, said Kim Amendola of NOAA. Sarthou said she was worried the activity around the well might disturb the whales.
Meanwhile, relatives of the missing waited for news.
Carolyn Kemp of Monterey, La., said her grandson, Roy Wyatt Kemp, 27, 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. He said his son told him 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.
The family of Dewey Revette, a 48-year-old from southeast Mississippi, said he worked as a driller on the rig and had been with the company for 29 years.
"We're all just sitting around waiting for the phone to ring and hoping for good news. And praying about it," said Revette's 23-year-old daughter, Andrea Cochran.
Those who escaped did so mainly by getting on lifeboats that were lowered into the Gulf, said Adrian Rose, vice president of Transocean.
Weekly emergency drills seemed to help, he said, adding that workers apparently stuck together as they fled the blast.
"There are a number of uncorroborated stories, a lot of them really quite heroic stories, of how people looked after each other. There was very little panic," Rose said.
Family members of two missing workers filed separate lawsuits Thursday accusing Transocean and BP of negligence. Both companies declined to comment about legal action against them after the first suit was filed.
The U.S. Minerals Management Service, which regulates oil rigs, conducted three routine inspections of the Deepwater Horizon this year - in February, March and on April 1 - and found no violations, MMS spokeswoman Eileen Angelico said.