In an hour-long public briefing from the National Mine Health and Safety Academy in Beaver, West Virginia, assistant secretary for mine safety and health Joseph Main and coal administrator Kevin Stricklin said the probable cause for the Upper Big Branch mine explosion was made worse by inadequate safety on the part of the mine’s then-owner, Massey Energy, as well as worker intimidation.

"There was some methane ignition that led to a massive coal dust explosion that could have been prevented," Stricklin said in the hearing, which was also teleconferenced.

He also noted that the situation was exacerbated by UBB’s inadequate ventilation as well as water spray issues.

MSHA’s initial findings go against those of Massey, which concluded after the blast that it was sparked by an unexplained natural gas inundation.

"There was a limited amount of methane and natural gas," Stricklin said.

Federal investigators, who briefed the family members of the victims in a private gathering in West Virginia Tuesday, also confirmed a second set of safety record logbooks at the Raleigh County operation.

The practice in itself is not a violation, but officials pointed to deceit as safety violations and hazardous conditions were not reported in a main book that was provided to miners and safety inspectors.

MSHA provided multiple comparison examples during the briefing, including the official log as well as a production book which illustrated the numerous hazards.

The rock dusting conclusion, officials said, had been drawn after the processing of over 23,000 photographs, hundreds of personal interviews and 18 video recordings.

Stricklin also said UBB had “chronic ventilation problems”, detailing a history of floor heaving and said that there had previously been three methane explosions at the mine – one in 1997 and two in 2003 and 2004.

"MSHA investigated and made recommendations, but there is no evidence they were acted on," he said, adding that the mine had been issued 17 violations in the year prior for inadequate rock dusting.

MSHA said Massey engaged in worker intimidation, threatening to fire those involved in shutting down operations deemed hazardous or for not meeting production targets. Training inadequacies were described, as were anecdotes of pre-notification of the presence of MSHA inspectors – a violation of federal law.

The federal agency’s final report, expected to exceed 200 pages, is expected to be released around October. A state investigation is ongoing and an independent study concluding a “man-made” cause for the blast was released in May.

Massey has since been acquired by Alpha Natural Resources, which did not make public comment Wednesday afternoon.

One group that responded to the MSHA briefing almost immediately was the nation’s largest industry union, the United Mine Workers of America, which focused on the duplicate logbooks Massey maintained at UBB.

"MSHA's revelation that there were two sets of books kept at the mine where information about safety issues were recorded demonstrates the utter contempt for mine safety and health laws that was pervasive throughout the entire management structure at Massey Energy,” president Cecil Roberts said.

"When mine management records continuing safety issues and hazards at the mine in production and maintenance reports, yet omits the same information from the official safety examination logbook, it confirms that management knew there were serious problems at the mine, yet chose to hide them from safety officials and the miners themselves. That's a crime, and punishment for those responsible for this cannot be too severe.”

He also noted that, while the UMWA was troubled by evidence of miner and foreman intimidation at the West Virginia mine, the revelation was not surprising.

“That was a long-term practice at Massey mines, part of a top-down culture at the company that put profits first and workers last,” Roberts said.

The union, which is acting as a miners’ representative, confirmed that its own investigation into the explosion was ongoing. It expects a report could be released by the end of the US summer.