President Barack Obama has ordered U.S. flags in West Virginia to be flown at half staff in honor of 29 miners killed in the Upper Big Branch mine explosion.

In a proclamation issued Monday, Obama said all U.S. flags in the state are to be flown at half staff until sunset on Sunday.

The proclamation comes as federal and state mine safety officials begin their investigation into the nation's worst coal-related mine accident in 40 years.

Crews are still working to recover the last of the victims. Recovery attempts have been hampered by high levels of hazardous gases.

State Office of Miners' Health Safety and Training spokeswoman Jama Jarrett says recovery of the last nine victims should be completed by late Monday afternoon.