An unusual murder mystery is playing out inside the Hazelton penitentiary in West Virginia.

The prime suspects — inmates — are already locked up. The victim was an aging mobster, someone plenty of people had reason to want dead.

But how James (Whitey) Bulger, the crime boss and law enforcement informant from Boston, came to be murdered just a few hours after arriving at Hazelton is still a puzzle.

Among the questions investigators are trying to answer, according to interviews with more than a dozen Federal Bureau of Prisons employees, lawyers with clients inside and relatives of inmates, are these:

Of at least four men sent to solitary confinement following the attack, which ones beat him to death?

Why was Mr. Bulger, a notorious informant in frail health, placed with other inmates in the general population?