Prosecutors say they may have made history by landing a conviction against ex-Massey Energy CEO Don Blankenship over workplace safety, though the misdemeanor charge fell far short of what they'd hoped for.

The conviction in West Virginia on Thursday capped a five-year investigation into an explosion at the Upper Big Branch Mine that killed 29 men, a painful chapter in Central Appalachian coal mining history.

Blankenship was convicted of conspiring to willfully violate mine safety and health standards, the least punitive of three counts he faced.

Jurors did not convict Blankenship of a more serious conspiracy charge to defraud federal regulators, which could have netted five years.

Blankenship was also acquitted of making false statements and securities fraud about company safety after the blast.

His defense plans to appeal.