Huntington, Utah ? Hundreds of rescuers struggled with falling rock and debris Monday in a desperate race to reach six coal miners trapped 1,500 feet below ground by a cave-in so powerful authorities questioned whether it was caused by an earthquake.

As the rescue stretched into the night, workers were unable to make significant progress and the initial effort was declared a failure.

“I’m very disappointed. That’s one step backward,” Robert E. Murray, chairman of Murray Energy Corp. of Cleveland, a part owner of the Crandall Canyon mine, told reporters at an evening briefing.

More than 16 hours after the collapse, searchers had been unable to contact the miners and could not say whether they were dead or alive. If they survived, Murray said, they could have enough air and water to last several days.

“They could have been struck by material and injured or killed, but we don’t know that yet,” he said.

Rescuers planned to spend the night bulldozing a road outside the mine to make way for a drilling rig that can punch holes large enough to improve ventilation and determine whether the men were alive, Murray said.

They also planned to continue drilling from inside and outside the mine, he said.

The mining crew was believed to be about four miles from the mine entrance. Rescuers were drilling into the mine vertically from the mountaintop and horizontally from the side, Murray said.

If they are able to open an old mine shaft, Murray said, rescuers believe they can get within 100 feet of where the men are trapped.

“The idea is to get a hole into where they are,” Murray said. “They could be in a chamber 1,000 feet long or they could be dead. We just don’t know right now.”

About a mile from the mine’s entrance, there was an all-day procession of trucks hauling heavy machinery headed toward the site to claw at rock and raw earth.

Relatives of the miners waited for news at a nearby senior center. Many of the family members don’t speak English, so Huntington Mayor Hilary Gordon hugged them, put her hands over her heart and then clasped them together to let them know she was praying for them, she said.

Outside the senior center, Ariana Sanchez, 16, said her father Manuel Sanchez, 42, was among the trapped miners. She said she cried when her mother told her the news, and declined further comment. No details were immediately available about the other miners.

The mine uses a method called “retreat mining,” in which pillars of coal are used to hold up an area of the mine’s roof. When that area is completely mined, the company pulls the pillar and grabs the useful coal, causing an intentional collapse. Experts say it is one of the most dangerous mining methods.