MONTCOAL, W.Va. -- Federal and state mine safety officials began searching the Massey Energy Co. mine for causes of the explosion that killed 29 miners last week and were expected to look closely at ventilation systems and safety procedures.

The discovery of the bodies of the last four missing miners over the weekend shattered the hopes of families in the Coal River Valley, where American flags were lowered to half mast and handmade signs offering prayers and condolences were taped to windows.

Hundreds of people gathered Saturday for a candlelight vigil outside the local elementary school where officials held briefings in what was the worst coal-mining accident in 40 years. West Virginia Gov. Joe Manchin has said he would ask for a national moment of silence at 3:30 p.m. Monday, a week after the mine accident.

The accident investigation could take as long as a year to complete, according to Mine Safety and Health Administration officials. Investigators are gathering samples from the mine and will soon conduct interviews with miners about the conditions at the time of the explosion and the mine's general safety practices.

On Monday, members of MSHA's investigation team, made up of about a dozen safety officials from the agency and two trial lawyers from the Labor Department's Office of the Solicitor, will hold their first meeting.

MSHA couldn't confirm Sunday if all of the 22 bodies still in the mine early Saturday had been removed, a process that requires mapping their location in the mine. The bodies have to be carried out by hand at least part of the way because damage to the mine prevents the use of a railcar in some areas, MSHA official Kevin Stricklin said.

All electrical equipment in the mine—such as cables and power-generation systems—will be tested by manufacturers and independent labs to see if they were functioning properly or if any might have caused a spark that ignited methane gas underground. Investigators also will look into the operation of ventilation systems to see why methane gas had built to explosive levels.

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