Efforts to reach six miners trapped underground in the US state of Utah since Monday have been suspended due to seismic activity, the mine owner says.
Robert Murray, chairman of the company which owns Crandall Canyon mine, stated that "we are back to square one underground," the AP news agency says.
No contact has been made with the men, some 1,500ft (457m) underground.
If the miners have survived, they have enough air and water to last several days, the mine company said.
Rescuers had been planning to drill through a mountainside to try to free the miners, and a large drilling rig was taken to the area, after a road was cut through the surrounding forest.
Another drill arrived by helicopter.
According to Mexico's consul in Utah, three of the miners are Mexican citizens.
CRANDALL CANYON MINE
Located in Manti-La Sal National Forest, 16 miles (25km) from Huntington
Uses the "retreat mining" method, which experts say is one of most dangerous
Coal pillars hold up area of mine's roof. When completely mined, pillars are pulled, causing intentional roof collapse
It is still not clear what caused the cave-in at the mine, 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 that 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.
However, Robert Murray of Murray Energy Corporation, the mine's owners, is still insisting that an earthquake caused the collapse.
"This was an earthquake, contrary to what others might have you believe," he said.
The miners are thought to be 3.4 miles (5.5km) from the entrance of Crandall Canyon mine.
Attempts were being made in a number of different ways to rescue the men but the mine company said it would take three days to reach them, if everything goes well.
Overnight on Monday, hundreds of rescuers at the Crandall Canyon mine, near the town of Huntington, drilled through rock and debris.
They had hoped to reach the miners via an adjacent, abandoned mine shaft, but were forced to withdraw after moving just 50ft (20m) because of "geological and geotechnical problems".
Relatives are waiting anxiously for news of the six men
"The families are doing well considering the circumstances," said Mr Murray, describing the incident as "a tragedy for them and for America".
Judy Bishop, whose cousin was among the trapped miners, told Reuters news agency the families were "hopeful but sombre".
"I always have hope. When you give up hope you give up life. I won't give up hope," she said.
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 have insisted they run a safe mine.