Naoma, West Virginia (CNN) -- Rescuers expect to find out by about midnight whether any of the four missing miners is still alive, West Virginia Gov. Joe Manchin said Friday night.
"This journey will be ending in about two-and-a-half, three hours," he said.
Authorities plan to notify the four miners' families first about the miners' fate.
"We'll go and give the news, whatever it may be," Manchin said. "We'll console and hug each other, and then we'll come down and give you the rest of the news."
Toxic air and smoke had forced rescuers to leave the mine earlier in the day, but nitrogen was poured in to replace the bad air, and the mine is no longer unsafe, Kevin Stricklin of the U.S. Mine Safety and Health Administration said Friday.
"Basically, we're very happy with the concentrations of gas," he said. "It is not explosive; it's not on fire; we have no smoke."
Each of the rescuers is carrying 70 to 80 pounds of equipment on their back, he said.
"We think now we can run for the face," Stricklin said.
The rescuers first plan to reach a refuge chamber and see whether it has been deployed. If it has not, they will search the "long wall face area" of the mine for three miners who were thought to have been in that area at the time of Monday's blast, he said.
Rescuers had earlier reached another refuse chamber but found that it had not been used. There are two such chambers in the mine.
Though hope of finding survivors was slim, "we're still focused on a rescue operation," Stricklin said Friday night. "After that, we'll focus on a recovery operation, and then, thirdly, we'll get into the investigative process to try to pinpoint exactly where the explosion occurred."
The cause of the blast is unknown, and state and federal officials have pledged a full investigation.
Earlier Friday, crews had reached a refuge chamber that had not been used, but the bad air forced them to evacuate before they reached a second chamber. The airtight chambers are stocked with enough food, water and air to keep 15 miners alive for four days.
"We're very confident that if they didn't make it to this refuge chamber, there's no way they could have survived," Stricklin said Friday.
Meanwhile, funerals for some of the miners began taking place Friday morning.
At the funeral for Benny Willingham, the Rev. Gary Pollard said the 61-year-old miner had three loves in life: God, his family and his job.
"He loved God so much that every day was a holiday; every meal was a buffet," Pollard said at the Mullens Pentecostal Holiness Church in Mullens, West Virginia.
Willingham, who was married for 33 years, was extremely devoted to his church, Pollard said. He had been a Christian for 19 years, he said.
And even though Willingham was set to retire soon, Pollard said he didn't know whether retirement would have suited him: He loved the work.
The funeral for Steven "Smiley" Harrah, 40, was also scheduled for Friday in Shady Spring, West Virginia.
Two others are set for later Friday, and one is scheduled for Saturday.
The explosion has prompted renewed questions about mine safety. Obama said Friday that "it's clear more needs to be done" to improve mine safety.
He will meet next week with Secretary of Labor Hilda Solis and a Mine Safety and Health Administration official to hear their initial assessment of the cause of the blast and their recommendations on steps the federal government should take to improve safety.
Richmond, Virginia-based Massey Energy Co., which owns the mine, said in a statement released Friday that it will conduct "extensive" reviews of the mine accident "to ensure that a similar incident doesn't happen again."
It said the mine has had less than one violation per day in inspections by the Mine Safety and Health Administration and added that that rate is "consistent with national averages."
"Most of the citations issued by MSHA to [Upper Big Branch] in the last year were resolved on the same day they were issued," it said. "The safety of our members has been and will continue to be our top priority every day."
One of the unaccounted-for miners and 18 of the dead were working in an area where long wall cutting was taking place. The technique uses a large grinder to extract the coal and creates large amounts of coal dust and methane, both of which are explosive.
The other three missing miners were believed to be about 2,000 feet away in a new development area of the mine.
CNN's Samira Simone contributed to this report.