US rescuers in Utah have drilled into a mine cavity where six miners are believed trapped to find no sounds of life and only low levels of oxygen.
But officials are refusing to give up hope of finding the men alive, saying they may have fled to another area.
No contact has been made with the miners, who have been trapped 1,500ft (457m) underground since Monday after a cave-in at Crandall Canyon coal mine.
Separately, three people died in a mine accident in southern Indiana on Friday.
The three construction workers died in a basket used to transport people along a 600ft (183m) air shaft being built at the Gibson County Coal mine, police said.
No other workers were missing after the incident, near the city of Princeton. The cause of the accident was still unclear, but police said no explosion had occurred.
'Common goal'
In the Utah mine, a 2.5-inch (6.4cm) drill pierced through the cavity on Thursday night.
CRANDALL CANYON MINE
Initial readings showed oxygen levels inside of over 20%, the level necessary to sustain life, but they were in fact taken from the bore hole and not the mine, Bob Murray, head of Murray Energy Corp, co-owner of the mine, later said.
When the drill was raised, oxygen levels fell to about 7%, which is not enough to support human life. There was also said to be no sign of carbon dioxide to indicate exhalation.
Rescuers are still working on drilling a wider 9in (23cm) hole through which a camera could be passed to check for signs of life, and via which water and food could be delivered.
Mr Murray said it could take at least another four or five days to bring the men out.
"We're optimistic, we're enthusiastic, we maintain our hope that we will reach our common goal and that is to rescue the miners," said mine safety expert Richard Stickler said.
Cause unclear
Rescue teams have been drilling round-the-clock through a mountain above the mine to reach the men, who include three Mexican citizens.
The group is thought to be 3.4 miles (5.5km) from the mine entrance, 140 miles (225km) from Salt Lake City.
Initially the collapse was blamed on an earthquake, but experts have since suggested the shafts might have caved in with enough force to register on seismographs.
This has led to speculation that mining procedures could have triggered the incident.
It has been claimed the workers were using a risky technique known as "retreat mining", where the last standing pillars of coal are pulled down and the roof is allowed to fall in.
But Mr Murray has rejected this, saying an earthquake caused the collapse.
US federal mining inspectors have issued 325 citations for alleged safety violations at the mine since January 2004. Of those, 116 were considered "significant and substantial" and likely to cause injury.
Experts have said the number of citations is not unusual, and the mine's owners say they run a safe mine.
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