Published: Apr 12, 2010 3:57 PM EDT

CHARLESTON, W.Va.  - Searchers went back inside a wrecked West Virginia coal mine Monday to pull out more bodies as federal investigators prepared to launch their probe of the explosion that killed 29 men in the worst U.S. coal mining disaster since 1970.

While some of the dead have been laid to rest, nine bodies remained in Massey Energy Co.'s Upper Big Branch mine in Montcoal. Efforts to recover them were stalled when high levels of volatile methane gas forced crews out of the mine Sunday.

After the tunnels were ventilated, one team entered around midnight Sunday and another went in about 5 a.m. Monday, state mine office spokeswoman Jama Jarrett said. A team of federal investigators is expected to arrive Monday to start trying to figure out what caused the explosion. Richmond, Va.-based Massey has been under scrutiny for a string of safety violations at the mine, though CEO Don Blankenship has defended the company's record and disputed accusations that he puts profits ahead of safety.

Authorities have said high methane levels may have played a role in the disaster. Massey has been repeatedly cited and fined for problems with the system that vents methane and for allowing combustible dust to build up. Mourning continued Monday, exactly a week after the explosion, with a wreath-laying ceremony at the state Capitol and a moment of silence planned for 3:30 p.m.

President Barack Obama on Monday ordered all U.S. flags in the state flown at half-staff until sunset Sunday. Virginia Gov. Bob McDonnell said he asked residents of his state to join the observance and continue looking for ways to help their West Virginia neighbors as they deal with a tragic time.

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