Air quality tests in the US coal mine where 13 workers have been trapped since an explosion on Monday are very discouraging, officials say.
Friends and relatives have been waiting more than 24 hours for news
Rescuers drilled a hole into the shaft and found high carbon monoxide levels, although the miners may still be alive.
"They could be in another location or they could be barricaded somewhere," the head of the mining company said.
A robot has been sent down to help rescuers decide if it is safe to go further into the West Virginia mine.
Earlier, rescuers had managed to move about two miles (3km) into the shaft.
The shaft collapsed after a blast at around 0640 (1140 GMT) on Monday. There has been no contact with the 13 men.
Hundreds of their friends and relatives have gathered at a nearby Baptist church to wait for news.
Tests showed carbon monoxide levels of 1,300 parts per million in the section of the mine where the 13 missing men are thought to be, the head of International Coal Group, the company that owns the mine, said.
Ben Hatfield added that the maximum level for sustaining life was 400 parts per million.
But, he said, miners were trained to barricade themselves in an area with clean air and wait for rescuers. They would not walk out if they thought there was danger, he said.
The miners have basic food and water, and are equipped with individual air purifying systems that would give them up to seven hours of clean air, officials have said.
International Coal Group recently took control of the Sago Mine, located in northern West Virginia, about 100 miles (160km) from the state capital, Charleston.
Are you in the area where the accident happened? Send us your eyewitness accounts.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.