Naoma, West Virginia (CNN) -- Efforts were under way Sunday to reventilate a sprawling West Virginia coal mine so crews can recover the bodies of 22 miners who died in an explosion Monday, authorities said. Seven bodies were removed soon after the blast.
While a state and federal investigation began immediately after the explosion at the Upper Big Branch South mine, officials cannot fully study conditions inside the mine until the bodies are removed, said Jimmy Gianato, West Virginia director of homeland security.
Reventilation was taking place on Sunday, and it likely will be 12 to 15 hours before recovery crews can go back inside, he said. The reventilation is necessary because the explosion destroyed most of the mine's ventilation systems, he said.
Gianato estimated it will take two days before all the bodies are removed and the full investigation can commence.
Hours after rescue efforts at the mine turned into a recovery operation Saturday, President Obama urged a thorough investigation into the cause of the explosion.
"This has been America's worst mining disaster in 40 years, and the toll on all West Virginians has been immeasurable, " Obama said. "We cannot bring back the men we lost. What we can do, in their memory, is thoroughly investigate this tragedy and demand accountability."
Crews on Saturday found the bodies of the last four miners who were unaccounted for after the blast. The death toll previously had stood at 25.
Nine of the bodies are located in a section of the mine where readings have showed high levels of gases, and recovery crews will need breathing gear to enter, said Jama Jarrett, spokeswoman for the West Virginia Office of Miners' Health, Safety and Training. The crews will mark where the bodies were found to help investigators, she said.
Jarrett said her office has a presence at the mine, and staffers will remain there throughout the investigation, conducting interviews with employees.
The mining disaster is the worst in the United States since 1972, when 91 miners died in a fire at the Sunshine Mine in Kellogg, Idaho.
West Virginia Sen. Jay Rockefeller, in an audio message released Sunday, offered prayers for those affected by the explosion and the families of the dead, and praised the perseverance of the rescue workers.
"You have our deepest sympathies, our deepest support," Rockefeller said, "and please know that all of us grieve for your loss."
"In our darkest hour, West Virginia, in fact, is a community that stands as one," the senator said.
The explosion has prompted renewed questions about mine safety. Obama said Saturday that "all Americans deserve to work in a place that is safe, and we must take whatever steps are necessary to ensure that all our miners are as safe as possible so that a disaster like this doesn't happen again."
Obama will meet next week with Secretary of Labor Hilda Solis and a Mine Safety and Health Administration official to hear their initial assessment of what caused the blast, along with their recommendations for 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."
Some have pointed to Massey's safety record in the wake of the blast. But the company said the mine has had less than one violation per day in inspections by MSHA and added that that rate is "consistent with national averages."
Most of the blast victims 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 gas, both of which are explosive.
West Virginia Gov. Joe Manchin said Saturday that even though the cause of the explosion is unclear, there needs to be a focus on better ventilation and on sensors to alert mine personnel when gas levels become dangerous.
"We are going to get to the bottom of this, because families should never have to pray, as they send their loved ones to work every day, that those loved ones will in fact return," Rockefeller said Sunday. "Our heroic coal miners have lost too many brothers and too many sisters, and now we must stop all of this."
CNN's Rachel Streitfeld contributed to this report.