HUNTINGTON, Utah (AP) - The desperate underground drive to reach six trapped miners was suspended indefinitely Friday after a catastrophic cave-in killed three rescuers inside a mountainside mine that keeps shaking.

The announcement from Richard Stickler, head of the federal Mine Safety and Health Administration, came after a cave-in Thursday killed three rescue workers and injured at least six others who were trying to tunnel through rubble to reach them. The initial cave-in occurred Aug. 6.

"Is there any possible way we can continue this underground operation and provide safety for the rescue workers? At this point we don't have an answer," Stickler said.

The cave-in at 6:39 p.m. was believed to be caused by what seismologists call a "mountain bump," in which shifting ground forces chunks of rock from the walls. Seismologists say such a bump caused the Aug. 6 cave-in that trapped the six men more than 3 miles 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.

Underground, rescuers had advanced only 826 feet in nine days. Before Thursday's cave-in, workers still had about 1,200 feet 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.

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