The families of 29 men killed in the 2010 explosion of Massey Energy's Upper Big Branch mine in southern West Virginia are facing what could be a pivotal day Tuesday, talking to both federal mine-safety investigators and federal prosecutors.

The U.S. Attorney's Office is holding a morning teleconference with the families and their lawyers to discuss what prosecutors will only call "significant developments" in the criminal investigation that grew from the deadliest U.S. mine disaster in four decades.

Then at noon, officials with the federal Mine Safety and Health Administration will present the families with the agency's final investigation report on the disaster. The public release of that report is set for later Tuesday in Beaver.

Gary Quarles, whose son Gary Wayne died in the explosion near Montcoal in April 2010, is hoping for criminal indictments.

"Somebody's got to pay for what's been done," he told The Associated Press late Monday.

Quarles, a miner for nearly 40 years and a former Massey employee, had been inside Upper Big Branch and knew his son faced bad conditions every day he went to work. But in the months since the blast, previous MSHA briefings and comprehensive reports by independent investigators and the United Mine Workers of America have revealed "it was worse than what we thought — a lot worse," he said.

Federal investigators have long blamed a combination of methane gas, coal dust and broken or malfunctioning equipment for the blast. The UMWA, the nation's largest coal miners' union, said last month that conditions were so dangerous that Massey — now owned by Virginia-based Alpha Natural Resources — should be prosecuted for "industrial homicide."

MSHA's final report is likely to include a list of specific violations that contributed, but the agency wouldn't comment on the report before its official release.

Alpha, which bought Massey and the Upper Big Branch mine in June, has said it's still reviewing the explosion. The state mine safety office, meanwhile, expects to complete its report by the end of January.

But all the reports so far agree on the mechanics of what happened: Poorly maintained machines cutting into sandstone created a spark that ignited both a small amount of naturally occurring methane gas and a massive accumulation of explosive coal dust. Malfunctioning water sprayers allowed what could have been a small flare-up to become an epic blast that traveled seven miles of underground corridors, doubling back on itself and killing men instantly.

All three also say the explosion could have been prevented or contained. Had the mine been sufficiently dusted with pulverized limestone to render the coal dust inert, the spark wouldn't have had the fuel to propagate.