The search for six miners was abruptly halted after a second cave-in killed three rescue workers and injured at least six others.
The setback Thursday came on the 11th day of the effort to find six miners who have been confined at least 1,500 feet (450 meters) below ground at the Crandall Canyon mine in Utah state. It was unknown if the six were alive.
"It just feels like a really hard blow to swallow after all we've been through the last week and a half and everyone trying to hope in their own individual way," Huntington Mayor Hilary Gordon said in telephone interview Friday with CNN's "American Morning."
All rescue workers were evacuated from the mine Thursday evening and work underground was stopped. Asked if the search would be suspended, Rich Kulczewski, a U.S. Department of Labor spokesman, said: "That's something to be determined."
The cave-in at 6:39 p.m. (0039 GMT, Friday) was believed to be caused by what seismologists call a "mountain bump," in which shifting ground forces chunks of rock from the walls with great force. Seismologists say such a bump caused the Aug. 6 cave-in that trapped the six men more than 3 miles (4.8 kilometers) inside the central Utah mine.
The force from the bump registered a 1.6 at the University of Utah seismograph stations in Salt Lake City, said university spokesman Lee Siegel. It was the 20th reading at the university since the original collapse, which registered a 3.9 on Aug. 6.
"These events seem to be related to ongoing settling of the rock mass following the main event," Siegel said Friday morning. "I don't think I'm going too far to say that this mountain is collapsing in slow motion."
The initial collapse led to the frenetic effort by rescuers to dig through the mine toward the men and drill narrow holes atop the mountain in an attempt to learn their whereabouts and perhaps drop food and water.
It was not immediately clear where the rescuers were working or what they were doing when Thursday's bump occurred.
Underground, rescuers had advanced only 826 feet (252 meters) in nine days. Before Thursday's cave-in, workers still had about 1,200 feet (365 meters) to go to reach the area where they believe the trapped men had been working.
Mining officials said conditions in the mine were treacherous, and they were frequently forced to halt digging because of seismic activity.