Last week Alpha completed all the commitments as part of a $US210 million ($A232 million) settlement to avoid any possible criminal charges over the UBB disaster, a deal necessary after Alpha acquired former UBB owner Massey in June 2011.

But this milestone does not impact the legal pursuit of key Massey officials.

"My primary focus continues to be on the investigation of misconduct by individual officials of the former Massey Energy,” US Attorney Booth Goodwin, who struck the settlement deal with Alpha, said according to the Charleston Gazette this week.

"That investigation has resulted in three criminal convictions to date, including the conviction of a former Massey division president – and it remains ongoing."

The attorney’s efforts have also been boosted by a considerable flow of information from Alpha over the past two years.

"Information from Alpha regarding safety-related changes at former Massey mines inherently discloses evidence about the operation of those mines under Massey, which bears directly on our ongoing investigation,” Goodwin reportedly said on Friday.

“Consequently, we are not releasing that information at this time."

An underground explosion, believed to have been further fuelled by the ignition of built-up coal dust, killed 29 miners in the West Virginia mining disaster on April 5, 2010.