Shares of Massey Energy (MEE) dropped nearly 10% on Tuesday after the company confirmed an explosion at one of its West Virginia mines on Monday.

The coal producer confirmed that at least 25 people had died in the company's Upper Big Branch Mine as a result of the explosion. It is the worst mine accident in more than 25 years.

"Tonight we mourn the deaths of our members at Massey Energy," said Don Blankenship, chairman and CEO of Massey, in a statement yesterday. "I want to offer my condolences to the miners' families who lost loved ones at Upper Big Branch."

The explosion comes as reports come out that Massey-owned mine in question was cited for numerous safety violations over the past several years. According to The New York Times,  the mine had 458 violations in 2009 alone.

In comments, Labor Secretary Hilda Solis said the federal Mine Safety and Health Administration will conduct an investigation into Massey's safety practices and "take appropriate action."

"Miners should never have to sacrifice their lives for their livelihood," Solis said in a statement.

The shares were recently off 8.4% to $50.10.