(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 cannot 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 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 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 doesn'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 doesn't happen again."