CHARLESTON, W.Va. -- United Mine Workers safety experts plan to release a report next week on their investigation into the worst U.S. coal-mining disaster in nearly 40 years. UMW officials have scheduled a press conference for Tuesday in Charleston to publicly release the report on the April 2010 explosion that killed 29 miners at Massey Energy's Upper Big Branch Mine in Raleigh County. Prior to the report's public release, the union will hold a private briefing with families of the miners who died. "Our charge is different from any other party to this investigation," said UMW President Cecil Roberts. "We don't have operational policies from which to divert attention. We don't have regulatory enforcement actions -- or inactions -- to explain away. We don't have lawsuits to defend against. "All we have are the surviving miners, their families and most of all, the families of the victims," Roberts said. "More than anyone, they deserve to know the entire truth about what happened to their loved ones and their co-workers. That's what we will report on Tuesday." Upper Big Branch was a non-union mine, but shortly after the disaster, miners working there designated the UMW to represent their interests in the investigation. The move gave UMW safety representatives a role in the on-site and underground portions of the government disaster investigation, but federal Mine Safety and Health Administration officials prohibited union officials from sitting in on witness interviews. MSHA chief Joe Main, a former union safety director, also refused the UMW's requests to conduct those interviews in public, and U.S. District Judge Irene Berger threw out a union lawsuit trying to force MSHA to conduct its investigation through public hearings. Labor Secretary Hilda Solis promised a series of public meetings about the disaster, but then backed off that idea when U.S. Attorney Booth Goodwin said the public events could jeopardize his ongoing criminal probe. During a U.S. Senate hearing six weeks after the disaster, the UMW's Roberts predicted that "investigations of the Upper Big Branch tragedy will show that safe mining practices were not followed at that operation and miners were being exposed to senseless dangers." So far, a report by special investigator Davitt McAteer and preliminary findings from MSHA agreed that the explosion involved an ignition of a small amount of methane gas that turned into a massive coal-dust blast because of Massey's poor safety practices. Investigators believe the ignition likely was sparked by worn-out longwall cutting teeth hitting sandstone on the longwall machine's shearer. They also believe that a coal-dust buildup underground sent what could have been a minor ignition into an explosion that rocketed in all directions, greatly increasing the damage and deaths. The McAteer team concluded the disaster was caused by Massey's failure to follow basic safety standards, and by a corporate culture that put coal production ahead of worker safety. McAteer and his team cited poor ventilation practices, illegal accumulations of highly explosive coal dust, and a failure to maintain water sprays and cutting bits on the longwall shearer. The McAteer report also criticized the West Virginia Office of Miners Health, Safety and Training and said the fact that 29 miners died in a mine explosion was proof positive that MSHA "failed its duty as a watchdog for coal miners." Alpha Natural Resources, which bought Massey in June, has said it is still reviewing the disaster. Reach Ken Ward Jr. at firstname.lastname@example.org or 304-348-1702.