The rescue effort to find six missing Utah miners was halted Friday when officials suspended the underground search after a second cave-in killed three rescue workers — one of whom was an MSHA inspector — and injured at least six others, according to media reports.

A team of mine-safety experts will review whether or not it is safe for rescue workers to return to underground rescue efforts, said MSHA’s Richard Stickler.

Rescue workers were evacuated from the mine Thursday evening after the cave-in. Three rescue workers remain in the hospital.

It is still not known if the six men trapped in the Aug. 6 collapse at the Crandall Canyon mine in central Utah are alive.

Utah Gov. Jon M. Huntsman Jr. called the rescue workers who died heroes, adding that worker safety must come first in the ongoing rescue operation.

Outside the mountain, crews were drilling a fourth hole for any sign of the six men, 1,500 feet below ground.

A "mountain bump" reportedly led to the cave-in at 6:39 p.m. Thursday with shifting ground forcing chunks of rock from the walls. Seismologists say such a bump caused the Aug. 6 cave-in that trapped the six men more than three miles inside the 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."

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 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.

Sources: and Associated Press