Goodwin has said May's guilty plea showed that the obstruction of federal Mine Safety and Health Administration inspectors "was a routine matter at Upper Big Branch."
Four investigations have concluded that Massey concealed problems at the mine through an elaborate scheme that included sanitized safety-inspection books and an advance-warning system.
Clay Mullins, whose brother Rex died in the mine, was in the courtroom and believed May should have gotten a longer sentence.
"I think it's a farce," Mullins said. "It's a pretty good deal for him, it sounds like. He admitted that he was guilty, admitted he altered records, gave advance notice to inspectors.
"Those 29 men put their faith in him as a mine foreman and a mine superintendent to provide them with a safe work place, and he was doing the opposite. He was putting production ahead of safety, production ahead of those men's lives. And it cost those men their lives."
May's sentencing came the same day that federal officials announced new rules aimed at improving safety at the nation's most dangerous mines by revising the way operators are designated pattern violators. The changes were proposed after the Upper Big Branch explosion.
Methane and coal dust fueled the explosion that was sparked by worn teeth on a cutting machine. It was allowed to propagate by clogged and broken water sprayers. Miners were killed instantly by the force of the blast that traveled along miles of underground corridors.
Goodwin said the investigation, however long it takes, "will continue until it's exhausted."
"It's still very much at the forefront and we're at a stage of the investigation where we're sifting through a large amount of information. We're being very careful," he said. "Unfortunately, that takes some time. My concern is making sure we do it right, that we get the individuals most responsible, if any out there, and hold them accountable. Really, we want to make sure something like Upper Big Branch never happens again."