Crews in Utah were forced to suspend their desperate underground search for six trapped coal miners on Friday after a cave-in killed three rescue workers and injured six.

"We have suspended indefinitely the underground portion of this rescue effort," Richard Stickler, head of the U.S. government's Mine Safety and Health Administration, told reporters.

Rescuers will continue to drill bore holes down through the top of the mountain to find the miners, who have not been heard from since a collapse on Aug. 6 and are thought to be located 1,800 feet (550 meters) underground.

If the miners were found, a larger hole would be drilled in from the surface in a long, slow effort, he said.

One of those killed and one seriously injured in Thursday evening's collapse were federal employees, Stickler said. The collapse raised fears that tunneling to try to find the men trapped in the central Utah mine was too dangerous to continue without further casualties.

"Yesterday we went from a tragedy to a catastrophe," Utah Gov. Jon Huntsman said outside the Crandall Canyon Mine, south of Salt Lake City, as he called for new efforts to make mining safer in his state and the country.

"We would envision doing an investigation in conjunction," with the Mine Safety and Health Administration, he said.

Co-owner Robert Murray, who has been the public face of the mine operating company, did not appear before the media on Friday. Stickler and other officials looked strained as they faced challenging questions from reporters about safety in the rescue effort.

'MOUNTAIN BUMP'

Officials in the tight-knit mining community struggled to cope with the tragic developments touching more families.

"Some (of these deaths) have affected me personally, in this last one very much so," Huntington Mayor Hilary Gordon said with tears in her eyes.

"Families that I love and know. I knew the other families, too, but this has been extremely hard."

Thursday's cave-in was called a "mountain bump," in which rock and coal erupts along the side of a tunnel under pressure from overhead rock. "When that energy gets released, it is like an explosion," Stickler said.

"Last night, the right rib exploded off of a coal pillar with tremendous force and knocked out all the ground support we had in place." Some of the rescuers had to be pulled from under coal and debris, he added.

Seismologists at the University of Utah said they recorded waves from the bump "consistent with further settling and collapse within the mountain."

It remains unclear what caused the first collapse. Murray has said it was triggered by an earthquake but geologists say it was the other way around and the collapse caused magnitude 3.9 seismic waves.

Controversy also rose over reports the miners were engaged in dangerous "retreat mining" when the shaft collapsed. Murray has denied such a technique was being used.

Retreat mining involves supporting a mine's roof with a column of coal, then removing that pillar and allowing the shaft to collapse as miners move to safety.