( CNN ) -- President Obama on Saturday urged a thorough investigation into a deadly explosion at a West Virginia coal mine that killed 29 people , hours after rescue efforts at the mine gave way to an operation to recover the dead . `` This has been America 's worst mining disaster in forty years , and the toll on all West Virginians has been immeasurable , `` Obama said of Monday 's blast at the Upper Big Branch mine . `` We can not bring back the men we lost . What we can do , in their memory , is thoroughly investigate this tragedy and demand accountability . '' Obama 's statement came just hours after rescue efforts came to a grim end after crews found the bodies of the last four miners unaccounted for in the explosion . Twenty-five people were previously announced dead . `` My thoughts and prayers are with the families of all those who were lost in this tragic accident , and my gratitude goes out to the rescue teams who worked so tirelessly and heroically to search for the missing , '' Obama said in his statement . The death toll makes the West Virginia mining disaster the worst in the U.S. since 1972 , when 91 miners were killed in a fire at the Sunshine Mine in Kellogg , Idaho . Of the 29 dead in the West Virginia blast , the bodies of 22 remain inside the mine . The cause of the blast is unknown , and state and federal officials have pledged a full investigation . 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 does n'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 mine 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 . '' 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 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 it 's not clear what caused the explosion , there needs to be a focus on better ventilation and on sensors to alert mine personnel when gas levels become dangerous . `` There was no way to protect them against this , '' he said . `` You just have to prevent it and make sure it does n't happen again . ''
President Obama calls for thorough investigation into mine blast . Four missing miners found dead ; death toll in West Virginia coal mine explosion now at 29 . Most of the victims were working in an area where long wall cutting was taking place .