The report said he was found unresponsive at 8:20 a.m. and was pronounced dead by the Preston County medical examiner.
Bulger's attorney, Hank Brennan who tried to appeal his life sentence all the way to the U.S. Supreme Court, declined to comment to the Boston Herald pending notification of Bulger's death by the Bureau of Prisons.
Bulger was previously relocated from a Florida prison to a facility in Oklahoma.
Prosecutors believe Bulger was responsible for more crimes during the 1970s, 80s and 90s, while he oversaw his criminal empire, than he was convicted of.
The Federal Bureau of Prisons online inmate log had listed Bulger as an inmate there early Tuesday morning, but by the afternoon he was no longer there.
The organized crime boss (pictured above in undated photos) had been convicted in 2013 of killing at least 11 people and was serving a life sentence at the time of his death Tuesday.
Bulger was later identified as the dead man, although officials released no details surrounding his sudden, violent demise. Bulger also served as the inspiration for Jack Nicholson's character Frank Costello in Martin Scorsese's 2006 film The Departed.
She was sentenced to eight years in prison for helping Bulger evade the law. It's unclear if authorities have determined who played a role in the murder.
The FBI had been searching for Bulger for 16 years before his arrest.
The 2015 film Black Mass, starring Johnny Depp, chronicled Bulger's years as an Federal Bureau of Investigation informant.