Aaron Hernandez is already on trial for one murder, and he could soon be on trial for two more.

Prosecutors are now asking a grand jury to indict Hernandez, a former star player with the New England Patriots, for the shooting deaths of two men outside a Boston nightclub last July, according to the Boston Globe. Investigators had previously said they were looking into whether Hernandez had any connection to that crime, but this is the first sign that they believe they have a real case against him.

"The case against Hernandez appears to be strengthening," a law enforcement official told the Globe.

More from the Globe:

The proceedings are conducted behind closed doors, so details of the case against Hernandez are unknown. But the fact that the case has gone to a grand jury indicates that the investigation of the killing of two Cape Verdean immigrants is now more narrowly focused on Hernandez. Safiro Furtado and Daniel Abreu were shot early on July 16, 2012, in the South End when someone in a sport utility vehicle with Rhode Island license plates pulled up next to their car and began firing. [Boston Globe]

Under Massachusetts law, grand juries must indict anyone charged with murder before they can go to trial.

Previous reports noted that a group of men including Hernandez got into a fight with Furtado and Abreu at the Cure Lounge in South Boston. Hours later, as the two victims sat idling at a red light, a silver or grey SUV pulled up alongside them and sprayed their car with gunfire.

Earlier this month, a Florida court filing revealed that police found an SUV matching the description of the vehicle wanted in connection to the double homicide at Hernandez's uncle's home in Connecticut. There was already speculation that Hernandez, who has been charged with the murder of Odin Lloyd and a handful of gun offenses, may have killed Lloyd to keep him quiet about the double homicide.

Hernandez has been held without bail since June 26, when he was charged with those crimes. Court documents released since then revealed that Hernandez slammed a door on the police when they first showed up at his home, and that an alleged accomplice claimed Hernandez told a third accomplice he fired the fatal shots that killed Lloyd.

Hernandez is due in court Wednesday.