HUNTINGTON, Utah - Another attempt to sink a video camera deep inside a coal mine where six miners have been trapped since a collapse a week ago yielded no signs of life, officials said Monday, frustrating crews trying to find the men.

The camera was paired with better lighting, but still only saw about 4.5 metres when it was lowered into the mine overnight, said Al Davis, who oversees Western operations for the federal Mine Safety and Health Administration. The images that came back included a distorted conveyor belt and an intact roof.

It was the latest disheartening news for rescuers and relatives of the missing men. A video camera lowered into the collapsed mine Sunday revealed equipment, but no sign of the miners. On Monday, crews were planning to drill a third hole in the hopes of finding them, but cautioned that the effort could take up to six days.

"I've accepted all possibilites," said an exhausted Cody Allred, the 32-year-old son of missing miner Kerry Allred.

It is still unclear what caused the mine to collapse. Bob Murray, head of Murray Energy Corp. and co-owner of the mine, has insisted it was caused by an earthquake but seismologists say there was no earthquake and that readings on seismometers actually came from the collapse.

One of the four miners who escaped alive said Sunday he didn't feel or hear a thing as the mountain shook and caved in, trapping six of his colleagues.

Tim Curtis was near the mine's entrance on Aug. 6 when he got a text message telling him of the collapse on his PED, or personal emergency device. The trapped men are believed to be about 3.4 miles from the mine's entrance.

"Where I was at, I felt nothing," Curtis said in an interview with The Associated Press. "It's just like you are here and three miles away are you going to hear a balloon pop?"

Curtis, a 33-year-old third-generation miner who works as a mine fire boss, or safety inspector, has worked 12-hour shifts every day since the collapse to aid the rescue effort.

Two holes have already been drilled about 550 metres down into the mountain containing the mine. The first is a 6-centimetre-wide hole that rescue crews initially believed drifted during the drilling process into a neighboring sealed chamber. A microphone dropped into that first hole heard no sign of the miners and air samples recorded an atmosphere of only 7 percent oxygen -- measurements similar to those known to exist in a sealed area of the mine and an oxygen level that would not sustain life.

The hole was later determined to be in an active work area, and rescuers were pumping air down the hole.

A second hole measuring nearly 23 centimetres allowed crews to lower a camera into a cavern that officials said showed a "survivable space." But images were limited and the camera was withdrawn to clear off one lens.

The second attempt to use the camera to get a glimpse of the missing men was hampered by poor lighting that limited the camera view to only about 4.5 metres into a 1.7-metres-high void at the bottom of the hole, far less than the 30 metres it's capable of viewing, said Richard Stickler, head of the Mine Safety and Health Administration.

Rescuers saw a tool bag, a chain and other items that are normally seen underground in a mine, he said. "We did not see any sign at all of any of the miners," Stickler said.

Searchers were set to begin drilling the third hole midday Monday, Davis said.

Despite the setbacks, rescue leaders said they were proceeding under the assumption the miners remain alive.

Curtis said the miners likely improvised an emergency plan when all four entry tunnels near their work site were blocked.

"It's just like your house, if it caught on fire and you have a plan to get out the window and that window is blocked," Curtis said. "That's what happened here."

Rescuers have been moving horizontally through the mine to try to reach the men. Their route was blocked about 600 metres from the men and rubble had been cleared from about 180 metres of the route, Stickler said.