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 can not 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 can not 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 does n'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 .
NEW : Officials hope to have all bodies removed by late afternoon Monday . Nine bodies remain in West Virginia coal mine after 13 removed on Sunday . The miners were trapped for more than a week before being rescued and taken out of the mine. President Obama to meet with safety officials over cause of the mine explosion .