CHARLESTON, W.Va. -- Government investigators and a crew from Massey Energy hope to re-enter the Upper Big Branch Mine on Wednesday to begin preparing to gather evidence in the probe of the worst U.S. coal-mining disaster in 40 years.Massey announced the plan Thursday in a news release issued following a conference call meeting between company officials and representatives of the U.S. Mine Safety and Health Administration and the state Office of Miners Health, Safety and Training.Investigation teams have been unable to get back into the Raleigh County mine for the last six weeks, citing high levels of toxic gases, the likelihood of fires smoldering underground and the possibility of secondary explosions."The plan is to send mine rescue teams underground on Wednesday," said MSHA spokeswoman Amy Louviere. "However, plans can change if the conditions underground take a turn."Louviere said MSHA experts will monitor air quality in the mine over the weekend, and that a final decision won't be made until Wednesday.Ron Wooten, director of the state mine safety office, said the most recent measurements of carbon monoxide and methane inside the mine looked good and indicate the mine atmosphere has stabilized in the last few weeks.Massey said that a 16-member team of federal and state officials and Massey employees would be the first to re-enter the mine. That team would consist of highly trained mine rescuers who would likely use self-contained breathing apparatus to examine the entire mine, and possibly rebuild any ventilation systems that are needed to make it safe for investigators.It's not clear how much of that type of work would be needed, in part because Massey has added several boreholes with large fans that are being used to sweep fresh air through the mine tunnels."The initial reentry will focus on monitoring the quantity and quality of the air in various locations underground," Massey said in a news release. "The results of this monitoring will dictate when additional entries into the mine to determine the cause of the explosion can begin."Twenty-nine miners died in the April 5 explosion. Investigators and mine safety experts believe the blast involved the ignition of methane and was made far worse by a buildup of highly explosive coal dust.Along with investigations by MSHA and the state, federal prosecutors are conducting a criminal probe of safety practices at Upper Big Branch.Reach Ken Ward Jr. at firstname.lastname@example.org or 304-348-1702.