The official said that in the interviews over recent days the FBI has been looking for any evidence that the company engaged in criminal negligence. The official spoke on condition of anonymity because the inquiry has not been made public.
The FBI declined to comment on the matter at its headquarters in Washington.
Massey's stock plunged 11 percent Friday.
Massey spokesman Jeff Gillenwater said the company is aware that investigators are interviewing witnesses but is "not aware of the nature of their investigation. We intend to cooperate in all phases of the accident investigation."
Prompted by a separate mine accident in Kentucky, President Barack Obama issued a statement saying his administration is taking steps to demand accountability for safety violations and to "strengthen mine safety so that all of our miners are protected."
The FBI and Labor Department investigation into the accident at the Upper Big Branch coal mine follows strong statements two weeks ago in which Obama criticized the company.
On April 15, the president asked the secretary of labor to work with the Justice Department "to ensure that every tool in the federal government is available in this investigation."
"Safety violators like Massey have still been able to find ways to put their bottom line before the safety of their workers -- filing endless appeals instead of paying fines and fixing safety problems," Obama said at the time.
Massey Energy's response at the time called the president's remarks "regrettable" and said that "unfortunately, some are rushing to judgment for political gain or to avoid blame."
Massey has dealt with criminal investigations after mine accidents before. In 2006, two miners were killed in a fire at Massey's Aracoma Alma No. 1 mine, also in West Virginia. Its subsidiary, Aracoma Coal Co., paid $4.2 million in civil and criminal penalties.
At a congressional hearing this week, Mine Safety and Health Administration director Joe Main told lawmakers that Massey has a "troubling record" when it comes to protecting its workers. He said Massey appeared to take a "catch-me-if-you-can" attitude toward workplace safety at the mine.
Mine safety officials have suggested that a preventable buildup of explosive methane gas and coal dust was the likely cause of the April 5 explosion. The Upper Big Branch mine was cited for more than 600 violations in the year and a half leading up to the accident, including problems with its methane ventilation system, dust accumulation and other issues.
Tony Oppegard, a mine safety advocate and former regulator who practices law in Kentucky, said prosecutors would look at whether Massey employees knew of hazardous conditions and decided not to fix them. Such conduct would be a criminal misdemeanor under federal mine laws.
Prosecutors also would likely check to see whether employees falsified records in which they are supposed to note any hazards in preshift reports or other reports filed during work hours. Falsifying such records is a felony.