According to Bloomberg, a source close to the probe said the two groups had divided the tasks involved and were meeting in the southern West Virginian cities of Charleston and Beckley.
The former jury is looking at allegations that some mine inspectors may have taken bribes from Massey employees, while the latter is investigating the explosion itself.
The source would not be named due to the secrecy of deliberations.
While Massey did not immediately respond to an ILN request for comment, spokesperson Jeff Gillenwater told Bloomberg that a comprehensive investigation by relevant law enforcement agencies was not uncommon.
“Massey has no knowledge of criminal wrongdoing,” he said.
He noted that the producer was “cooperating with all agencies that are investigating the tragedy” at the Raleigh County longwall operation.
“Massey does not and will not tolerate any improper or illegal conduct and will respond aggressively as circumstances warrant.”
According to federal data, a total of 247 interviews with those with knowledge of UBB had been conducted as of October 27.
At the state level, witnesses and about 15 Massey upper management employees were subpoenaed, though the latter group pled their Fifth Amendment right not to provide potentially self-incriminating evidence.
Various news outlets, including Reuters, have identified six individuals in the Massey group as UBB mine manager Wayne Persinger, Massey safety vice-president Elizabeth Chamberlin, UBB mine foreman Rick Foster, UBB mine superintendent Gary May, UBB chief electrician Rick Nicolau and Massey employee Jamie Ferguson.
The US Justice Department is conducting an investigation in addition to probes by the West Virginia Office of Miners’ Health, Safety and Training, and the US Mine Safety and Health Administration.