Sen. Arlen Specter of Pennsylvania, the ranking Republican on the Senate Appropriations subcommittee that oversees labor issues, says the panel will compel Murray to "come into" the committee's offices because he has failed to appear despite assurances that he would willingly testify. The committee authorized the subpoena earlier this month but has not issued it.

"We're going to get to the bottom of what went on there," Specter said in a recorded statement, "but [Murray] is an indispensable witness, and, candidly, he really flouted the responsibility and authority of the United States Senate to have his testimony to find out what happened so we could do our utmost to prevent future occurrences."

Murray, who has faced stinging criticism for his comments and behavior in the aftermath of the tragedy, had been invited to speak before the Appropriations Committee in September but he declined. Three congressional panels are probing the disaster at the rural Utah mine that left six miners entombed in the mountain and three would-be rescuers dead.

Murray Energy responded with a statement.

"We are not aware of any subpoena having been issued. We remain committed to a fair inquiry and continue to cooperate with the appropriate authorities," it said.

The House Education and Labor Committee has requested Murray turn over a slew of documents related to the Crandall Canyon mine, which was using a potentially dangerous type of mining, known as retreat mining or pillar pulling. Six miners were trapped in the mine after pressure from the mountain caused walls of coal to explode.

The House committee issued the first subpoena related to the Utah tragedy when it compelled the Labor Department to turn over a load of documents on mine safety across the country and specifically anything dealing with the approval of the mine plan for Crandall Canyon.

The Labor Department says it has turned over some 15,000 pages of documents but missed the subpoena's deadline of Oct. 9 because it said it would be impossible to comply with the request in such a short amount of time.