A Facebook page that sprung up shortly after the blast and quickly grew to tens of thousands of members listed condolences, prayers and outrage at mine safety.
"My father was a coal miner in WV in the 1930s," one poster wrote. "He told me of running out of a coal mine as fine, silty dust trailed behind him. It appears mining is NO safer now than it was then!"
Massey Energy is one of the nation's largest coal producers, but has had a spotty safety record. The company paid out millions of dollars in fines last year alone after admitting to repeated safety violations.
Manchin said an investigation would come later.
"I don't know what happened," Manchin said. "We're going to find out and do everything in our power to never let this happen again."
ABC News' David Muir, Cleopatra Andreadis, Desiree Adib, Lee Ferran, and The Associated Press contributed to this report.