The Utah mine where six workers were entombed after a thunderous collapse has been sealed, leaving expensive mining machinery inside, federal regulators said Tuesday.

Richard Gates, the lead investigator for the federal Mine Safety and Health Administration, made the disclosure in a briefing for a state commission that is looking into whether Utah should return to the business of regulating coal mines after a 20-year hiatus.

Kevin Stricklin, MSHA's administrator for coal-mine safety, said Cleveland-based Murray Energy Corp. sealed the mine's three main passageways with walls of concrete block in early October.

He said the decision did not represent any loss of hope for recovering the men's bodies, although that appears to remain a distant possibility.

Families of the six dead miners were notified. The walls can be removed if there's a new effort to recover the bodies, Stricklin said.

The miners were trapped in a cave-in Aug. 6. Ten days later, another collapse killed three people and injured six others trying to dig their way toward the trapped men.

Federal safety officials refused to share any preliminary findings with the Utah Mine Safety Commission headed by Scott Matheson, a law professor and former U.S. attorney.

Gates declined even to estimate when he would issue a report, but said it wouldn't take longer than the 18 months he spent looking into the deadly explosion at West Virginia's Sago Mine in January 2006.

In an interview, Gates said about 10 people with knowledge of the Utah disaster were refusing to cooperate in the investigation. He refused to characterize these people in any way, but said some of them were taking direction from superiors.

Also Tuesday, Stricklin disclosed that his agency had rescinded permission for deep-underground retreat mining at five Utah mines. Retreat mining involves yanking supporting pillars of coal from the mine, allowing the roof to collapse as miners and equipment work their way out. Utah's has eight active coal mines, but the other three did not have approval for this method.

Stricklin said his engineers are trying to determine if retreat mining can be done safely in Utah, where the geology under mountain ranges is fraught with danger. The ban applies to retreat mining more than 1,500 feet underground and could be lifted after the technical review, he said.

MSHA had approved retreat mining at Crandall Canyon, but mine co-owner Bob Murray has insisted it had nothing to do with the disaster.

Gates, meanwhile, displayed a series of grim photographs of the collapsed Crandall Canyon mine.

The pictures showed splintered chunks of coal packed tightly in tunnels, deformed roofs and caved-in walls. One photograph, taken after the collapse that killed three rescuers, showed only the bumper of their continuous mining machine buried in loose rubble — still 2,000 feet from the presumed location of the trapped miners.

But it was a photograph of a March 10 collapse — five months before the disaster — that brought the danger into ominous focus. Stricklin said he was startled by the devastation in the mine's north section. The company abandoned that section for the part of the mine that collapsed in August.