BOSTON — Aaron Hernandez ambushed and shot to death two men after a chance encounter inside a Boston nightclub, prosecutors said Thursday as they announced new murder charges against the former NFL star, who was already awaiting trial in another shooting death.

The victims in the 2012 killing, Daniel de Abreu and Safiro Furtado, were shot to death in a car as they waited at a red light on a July night in Boston’s South End neighborhood.

Suffolk District Attorney Daniel Conley would not elaborate on what happened in the nightclub, other than saying that the encounter “triggered a series of events that ended in the murders.” But he said that after the men left, Hernandez followed in an SUV, then pulled up alongside the vehicle and fired multiple shots from a .38-caliber revolver into the passenger’s side, killing de Abreu and Furtado, and wounding a third man. Two other passengers in the car were uninjured.

Conley said there was no evidence that Hernandez knew the victims before that evening.

Weeks after the double shooting, Hernandez signed a five-year deal worth about $40 million with the New England Patriots and went on to play 12 games as a tight end that season. A spokesman for the Patriots said the team had no comment.

Hernandez is already awaiting trial in the June 2013 shooting death of 27-year-old Odin Lloyd near Hernandez’s North Attleborough home and is being held without bail after pleading not guilty to murder in Lloyd’s death.