(CNN) -- Former New England Patriots tight end Aaron Hernandez is being investigated by a Massachusetts grand jury for possible gun trafficking, a source with knowledge of the investigation told CNN on Monday.
The news comes one day after Massachusetts State Police served one of Hernandez's former college teammates, Mike Pouncey, with a subpoena related to the investigation, sources confirmed to CNN.
After the Patriots defeated the visiting Miami Dolphins 27-17 at Gillette Stadium, police officers in suits gave Pouncey, a center for the Dolphins, a piece of paper in a hallway between a locker room and the Dolphins' bus, Sports Illustrated first reported.
The gun trafficking investigation deals with illegally transporting weapons across state lines and, in part, involves the discovery of an assault-style rifle hidden in a vehicle rented in Florida and later found in Massachusetts, according to a separate law enforcement source.
CNN requested comment from the Dolphins, the NFL, the Massachusetts State Police and the district attorney's office prosecuting the Hernandez cases in Bristol County, Massachusetts. Representatives from each declined to comment.
Hernandez, 23, was charged in August with first-degree murder in the June shooting death of his friend Odin Lloyd. The grand jury investigating the allegations of gun trafficking is the same one that handed up the murder charge, a source said.
Pouncey, 24, has not been charged in any possible wrongdoing and was asked to appear as a witness. He and his twin brother, Maurkice, both attended the University of Florida with Hernandez.
After Hernandez was arrested in June after the execution-style killing of Lloyd, a photo of the Pouncey brothers at a night club wearing baseball caps that read "Free Hernandez" was posted on Twitter July 14.
Maurkice Pouncey, a center for the Pittsburgh Steelers, later posted a statement, apologizing and saying it "made light of a serious situation." According to Sports Illustrated, a Steelers spokesman declined to comment when asked whether Maurkice Pouncey had been served. The Steelers will play at Gillette Stadium on Sunday.
Hernandez was arrested in late June, 10 days after authorities say Lloyd, a 27-year-old semi-pro football player, was killed in North Attleborough, Massachusetts.
Authorities have said that Hernandez, Ernest Wallace and Carlos Ortiz picked Lloyd up from his Boston apartment in a rental car shortly before he was found shot to death on June 17 in an industrial park not far from Hernandez's home.
Hernandez pleaded not guilty in September to a charge of first-degree murder. Wallace and Ortiz have pleaded not guilty to charges of being an accessory to murder after the fact.
Hernandez is also under investigation by a different grand jury in Boston looking into the unsolved 2012 double homicide of Daniel Abreu and Safiro Furtado outside a Boston nightclub.