Reaching six coal miners trapped when an underground shaft in Utah collapsed Monday may take several days, and it still isn't known whether the search will prove to be a rescue or a recovery operation, according to news reports.

The most promising approach route to the miners at the Crandall Canyon mine near Huntington, Utah, is “impassable,” Robert Murray, chief executive of mine owner Murray Energy Corp., said in a televised briefing late Monday. Two rescue teams trying to get within 55 feet (16.5 meters) of the miners were “driven out,” he said. No rescuer was injured.

That approach would have allowed rescuers to confirm today whether the workers are safe. Rescue teams are attempting a more time-consuming path that involves breaking the seal of an old mine, Murray said.

It could take two to three days to reach the workers that way, CNN reported. The six haven't responded to radio calls from rescuers, Mike McKown, a spokesman for Murray, based in Pepper Pike, Ohio, said in a phone interview.

The Utah teams never got closer than about 1,700 feet to the workers Monday, mine officials said. Progress in clearing materials from the site was “fairly slow,” mine operator Doug Johnson said. Rescuers set up a mechanical system to remove coal more efficiently, he said. About 200 people, including four six-person rescue teams were on site.

The miners were located about four miles (6.6 kilometers) from the entrance of the mine. Each had a tank that holds an hour's worth of oxygen, and there may be additional units in the section where the workers are trapped, Johnson said.

The force of the collapse was picked up by seismographs, prompting some initial reports that the accident was caused by a weak earthquake.

Seismic activity near the mine may have been triggered by a roof collapse within it, rather than a separate earthquake causing the roof to fall, said Walter Arabasz, director of the University of Utah seismograph stations. The rumblings he observed were “consistent with a mine-type collapse.”