State police sources said the gun found in June during an arrest was the same weapon used to kill two men in July 2012, WSHM-TV, Springfield, first reported Monday.
Police said the investigation into that case was still open and they thought Hernandez could be tied to it.
Hernandez is being held without the bail on first-degree murder charges in the death of Odin Lloyd.
Jailene Diaz-Ramos, 19, of Bristol, Conn., was arrested June 21 following an accident. State police found a .38 caliber Smith and Wesson revolver while inventorying her car, WSHM said.
State police sources told WSHM ballistics testing matched the gun positively with bullets found at a double homicide in South Boston and Hernandez now was being investigated for a possible link. Authorities said they think a sport utility vehicle towed from the home of Hernandez was used in that killing.
During booking, Diaz-Ramos told police the gun belonged to a friend she called "Chicago," WSHM said. She said she gave friends a ride to work and they were football players.