Naoma, West Virginia (CNN) -- Recovery crews worked in a West Virginia coal mine Monday, hoping to bring out by late afternoon the last nine bodies remaining there out of the 29 miners who died in an explosion a week ago, officials said.
Authorities redirected airflow in the mine to decrease methane levels that had impeded recovery efforts on Sunday, said Jama Jarrett, spokeswoman for the West Virginia Office of Miners' Health, Safety and Training. Thirteen bodies were removed Sunday, and rescue teams re-entered the mine around midnight.
Officials predicted the remaining nine bodies would be removed by 4 or 5 p.m. Monday, Jarrett said.
While a state and federal investigation began immediately after the blast 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.
The U.S. flag will be lowered to half-staff at all federal buildings in West Virginia for a week to honor the miners, according to a proclamation signed Monday by President Obama.
Hours after rescue efforts at the mine turned into a recovery operation Saturday, Obama urged a thorough investigation into the cause of the explosion, saying, "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.
The mining disaster was 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."
Obama plans to meet 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 the 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.