A disastrous cave-in caused by a seismic "bump" has killed three rescue workers trying to reach six trapped Utah miners, officials said early Friday.

Six other workers have been injured and the officials said they are considering suspending the rescue effort.

"It is believed that the accident was caused by a bump. … We are in the process of doing a head count to ensure that everyone is accounted for," said Dirk Fillpot, spokesman for the Mine Safety and Health Administration.

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Yellow ribbons line a bridge near the command post at the entrance to the Crandall Canyon Mine on Thursday. ((Rick Bowmer/Associated Press))

Thebump, whichcommonly refers to pressure inside the mine that shoots coal from the walls with great force, reportedly occurred at about 8:30 p.m. ET at the Crandall Canyon mine. The same kind of bump caused the earlier cave-in, seismologists said.

"All rescue workers have been evacuated from the mine. Nine rescue workers were injured in the accident. One of those suffered fatal injuries," Dirk Fillpot, a spokesman for the federal Mine Safety and Health Administration, said early Friday. Officials later confirmeda second and third fatality.

It was a shocking setback on the 11th day of the effort to find the six trapped miners. It's unknown whether themen are still alive, or whether they even survived the initial collapse.

Rescuers have said they are disheartened by the pace of their efforts.

Underground work 'disappointingly slow'

"The underground effort is going disappointingly slow," mine owner Bob Murray said at a news conference outside the collapsed coal mine earlier Thursday. "It's not going as rapidly as we hoped."

Seismic activity in the mine, located on a mountain near Huntington, caused landslides and forced rescuers to stoptheir work Wednesday night because of fears for their safety.

Seismic activity struck again Thursday morning, delaying plans to drilla fourth bore hole into the mine by at least several hours.Cameras and microphones will be dropped down the narrow hole, travelling hundreds of metres into the mine to search for signs of life.

The miners have beenmissing since a collapse Aug. 6 at the Crandall Canyon Mine, about 225 kilometres south of Salt Lake City. It is believed the miners are 457 metres underground.

Intact chamber found in mine

Murray said despite the frustrations, there have been hopeful developments in the search.

On Wednesday night, video images from the third bore hole showed anintactmine chamber, with its roof and floorin placeand its ventilation system unharmed.Water dripping into the chamber would be drinkable, Murray said.

"The good news is we have found a cavity where they could be alive," Murray said, showing the dimly lit video of the cavity to reporters.

Airquality samples taken from thechamber showed oxygen levels of about 16 per cent, said Richard Stickler, head of the U.S. Mine Health and Safety Administration.

Normal oxygen levels are 21 per cent, but 15 per cent is high enough to support life, although it could cause people to have elevated heart and breathing rates, Stickler said. Previous readings have found oxygen levels in the mine as low assix per cent.

Noises heard

Noises were also detected Wednesday night by equipment used to monitor vibrations in the mine, Stickler said, but he warned it could be rocks breaking or even animals.

"We saw some indication of noise for a period of about five minutes that we had not seen before," said Stickler, who joined Murray at the Thursday news conference.

Murray said efforts are moving slowly ona wider hole being dug horizontally into the mine that could allow rescuers to reachthe miners. After nine days of work,the hole is only 250 metres long, with an estimated 365 metres left to go.

If seismic activity lets up, rescuers might be able to dig the hole at a rate two or three times faster than what they've managed thus far, Murray said.

With files from the Associated Press