The Upper Big Branch coal mine, where 29 people were killed in a blast two years ago Thursday, will be permanently sealed by the summer.

Crews will seal with concrete the portals that allow entry to the mine, plug boreholes and cap mine fan shafts, said the mine's new owners Alpha Natural Resources.

"Though two years have passed, everyone still has vivid memories of the tragedy and the suffering the miners' families endured," said company Chief Executive Officer Kevin Crutchfield. "For all of us in the mining industry, it is a solemn reminder of why we must always put safety first in everything we do at work and at home."

The explosion at the West Virginia mine on April 5, 2010, was the deadliest U.S. mine disaster since 1972, when 91 men died in a fire at the Sunshine Mine in Kellogg, Idaho.

In a December report,the Mine Safety and Health Administration found a methane ignition that set off flammable coal dust was the immediate cause of the 2010 explosion.

But it also blamed the "unlawful policies and practices" of then-mine owner Massey Energy Co., which it said "promoted and enforced a workplace culture that valued production over safety."

Alpha Natural Resources bought Massey in 2011 and has agreed to a $209 million settlement to avoid prosecution. The deal includes payments of $1.5 million to each family that lost a member in the blast.