TMZ reports:

Aaron Hernandez ATTACKED another inmate at Bristol County Jail earlier today … and we’re told he beat the guy up pretty good … sources tell TMZ Sports.
Sources tell us the former New England Patriots tight end — who is usually segregated from the general population — was allowed to take a walk in an isolated hallway … but somehow came into contact with another inmate.
We’re told Hernandez recognized the other inmate and launched into an attack — beating the other man up pretty badly.
Sources tell us … Hernandez and the other man had been beefing all day long. One source says the other guy had been harassing Aaron nonstop.