CHARLESTON, W.Va. -- Federal regulators are set to release next week the report of their 20-month investigation into the worst U.S. coal-mining disaster in nearly four decades.The U.S. Mine Safety and Health Administration has scheduled a media briefing for 3 p.m. Tuesday at its training facility outside Beckley. A briefing for families will be held at the same facility, starting at noon that same day.MSHA is expected to release the findings of its probe and to issue any citations related to the April 5, 2010, explosion that killed 29 miners at the Massey Energy operation in Raleigh County.MSHA is not ready to release the findings of an internal review examining the agency's oversight at Upper Big Branch prior to the disaster.Historically, MSHA has released internal review reports in the weeks or months after their official investigation findings. Agency officials released internal reviews on the 2006 accidents at Sago, Aracoma and Kentucky Darby all at the same time in 2007, reducing negative media coverage to one day.So far, MSHA has released no timeline for finalizing the Upper Big Branch internal review report, or for resuming the schedule of public hearings initially promised by Labor Secretary Hilda Solis.Obama administration officials called off the public hearings for fear that public testimony about the disaster would impede the ongoing criminal investigation. U.S. Attorney Booth Goodwin's probe has netted two convictions, but neither involved charges directly related to the cause of the explosion.MSHA has already issued preliminary findings that mirrored those of special investigator Davitt McAteer, who found 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.A separate report by the United Mine Workers union mostly agreed with those findings. Union officials called the disaster "industrial homicide," and also cited a specific -- and allegedly illegal -- ventilation change by Massey that led to airflow problems and a methane buildup linked to the initial ignition.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.