The inmate suspected in the murder of Whitey Bulger is Fotios "Freddy" Geas who is serving a life term on a racketeering conviction involving, among other things, the 2003 slaying of Genovese capo Adolfo Bruno in Springfield, MA as reported by the Daily News:
It's unclear if Geas carried out the killing or served as an accomplice, but his attorney said the killer would likely never reveal the details."He wouldn't rat on anybody," David Hoose said, "and he had no respect for anyone who would."
Freddy Geas was an enforcer for Genovese capo Anthony "Bingy" Arillotta, and Bingy later flipped to testify at the 2011 federal racketeering trial against Freddy, his brother Ty Geas and Arthur Nigro who then was the acting boss of the Genovese family.
Further reading that may be of interest: