The U.S. Air Force flew in sonar equipment overnight to Grand Junction, Colo., as part of a process to search for signs of life in the mine. At some point today, it's expected that dynamite charges will be set off some distance away from the six remaining miners — who have been trapped by a collapse in the mine since just before 3 a.m. Monday.

If they become trapped, miners are trained to listen for the dynamite charges and then respond by banging on the ceiling of the mine with the hope that the sonar will pick up their vibrations. If the miners respond, it will be the first indicator that one or more of the miners survived the collapse.

Inside the mine, Murray said rescuers were only able to progress 310 feet toward the trapped miners.

Overnight, rescuers had to get out of the mine after a pressure-shifting "bump" was felt inside the shafts.

"It's a setback for us," said Al Davis with the federal Mine Safety and Health Administration. "We had to discontinue moving more process, more material."

In a news conference this morning, Murray said he is frustrated that efforts to reach the men have gone so slowly. The mine's owner said it could be days before they are able to reach the miners, but vowed to not stop until the men are free.

"The Lord has already decided if they're alive or dead and they were killed in the concussion of the earthquake," Murray said. "But it's up to Bob Murray and the others to get to them."

Rumbling about earthquakes

During the news conference, Murray repeatedly insisted that the mine collapse was caused by an earthquake in the area, not a seismic event inside the mine. Scientists at the University of Utah Seismograph Stations have suggested the collapse of the mine registered 3.9 on the Richter scale.

"This was caused by an earthquake," Murray told reporters. "It was a natural disaster."

Murray held up a University of Utah Seismograph Stations report that he said proves his stance. He said the epicenter of the 3.9 magnitude quake was too far away from where the miners were trapped to say, as it's been reported, that a mine collapse caused the earth to rumble.

However, seismologists at the University of Utah said preliminary reports indicated that the seismic "event" was caused by the mine collapse.

"The preliminary results are consistent with a mine collapse as the source of the seismic wave that we recorded," said seismologist Relu Berlacu. "However, at this time we don't rule out the event of an earthquake."

Berlacu said they did record several "aftershocks" measuring up to 2.0 on the Richter scale within the area, but said that could also be consistent with a mine collapse.

The Mine Safety and Health Administration said there will be an "exhaustive" investigation to determine what caused the collapse.