American football star Aaron Hernandez, already behind bars for one alleged homicide, was indicted Thursday on two more murder counts, US prosecutors said.

A grand jury in Suffolk County, Massachusetts, indicted Hernandez in the ambush style, 2012 double-shooting of two men killed as they sat in a parked car.

Prosecutors said the former player with the New England Patriots opened fire on the men in Boston's South End neighborhood after an altercation at a night club.

"This case is about two victims who were stalked, ambushed and senselessly murdered on the streets of the city they called home," Suffolk County District Attorney Daniel Conley told a Boston news conference.

Conley said the murder victims, Daniel de Abreu and Safiro Furtado, "were ambushed and executed" by Hernandez.

"When the victims' car stopped at a red light... Aaron Hernandez fired a .38 caliber revolver multiple times" into the men's car," he said.

"Today's indictments reflect nearly two years of investigation" by law enforcement, said Conley, adding that three other people in the victims' car at the time survived the shooting.

Authorities said additional charges could be filed against Hernandez, who has pleaded not guilty to charges linked to the June 2013 murder of another man, Odin Lloyd, in a Boston suburb.