Naoma, West Virginia (CNN) -- Federal authorities were determined late Thursday to resume the search in a West Virginia coal mine where four miners may be trapped after a deadly explosion earlier in the week.
Kevin Stricklin of the U.S. Mine Safety and Health Administration said rising barometric pressure in the area has helped reduce levels of dangerous gases in the Upper Big Branch mine. Stricklin said air sampling would continue every 15 minutes.
He said if the trend reverses and gas levels go back up, crews were ready to pump nitrogen in to neutralize the air and reduce the chances of an explosion.
"If the trend is continuing downward, everyone is in agreement to send rescue teams back into the mine," Stricklin said.
Rescuers had been using high-pressure fans to pull the toxic air through 1,100-foot holes to the surface Thursday.
At least 25 miners died in Monday afternoon's explosion at the Upper Big Branch Mine in southern West Virginia, while four others remain missing and two were injured.
Rescue crews had been pulled from the mine Thursday morning because deteriorating air quality posed the threat of a new blast, officials said.
Manchin said Thursday there was no change in the two injured miners' conditions. One is in intensive care, but the other is doing well, he said.
While 32 rescuers working in four teams were in the mine, officials began to get deteriorating readings on air coming out of a borehole, Kevin Stricklin of the U.S. Mine Safety and Health Administration told reporters earlier Thursday.
The readings showed potentially explosive levels of methane and hydrogen and high levels of carbon monoxide, he said.
Authorities have acknowledged it's unlikely the four missing men are alive, but they refuse to give up hope.
Manchin said he hopes the four were able to reach and deploy one of the mine's two airtight rescue chambers, which were stocked with enough food, water and air to keep 15 miners alive for four days. Officials hope to determine whether either rescue chamber has deployed by lowering a camera through the second hole.
"If they're deployed, more than likely someone's in them," Manchin said. "That means we have a chance of that miracle." But he added, "If they're not deployed, we know that our chances are diminished tremendously."
In an interview with CNN, Manchin noted that the mine was outfitted with gas sensors to alert mine personnel when levels become dangerous.
"In gaseous mines, you want to monitor and you want to have good ventilation," he said. "If that has worked before, whenever the levels got out of control they pulled them out and stopped, why did it not happen this time?"
Earlier Thursday, West Virginia congressman Nick Rahall said a "sliver of hope" remained. "But I think everybody knows the reality, including the family, that the odds are stacked against us tremendously."
Rescuers got within 500 feet of one of the rescue chambers before having to turn back Thursday, said J. Christopher Adkins, chief operating officer of Massey Energy, the mine's owner. Adkins said he believed the rising levels of the gases were linked to a storm front moving through the area, which caused a drop in barometric pressure.
One of the four unaccounted-for miners and 18 of the dead were working in an area where longwall cutting was taking place. The technique uses a large grinder to extract the coal. It creates large amounts of coal dust and methane, both of which are explosive.
The other three missing miners are believed to be about 2,000 feet away in a new development area of the mine.
Thirty-one men were working in the mine at the time of the blast Monday afternoon, Adkins said. The bodies of seven miners have been removed.
The cause of the explosion is unknown, and state and federal officials have pledged a full investigation.
The White House announced Thursday that President Obama will meet next week with Secretary of Labor Hilda Solis and an MSHA 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.
CNN's Rachel Streitfeld and Samira J. Simone contributed to this report.