A teenager with no fixed address was held without bail today after a Suffolk County homicide prosecutor said he was responsible for attempted robberies that led to the shooting death of 36-year-old Ciaran Conneely and the non-fatal shootings of two other men last year, District Attorney Daniel F. Conley announced.

JOHN GRAHAM (D.O.B. 12/15/94) was arraigned today on charges of first-degree murder, armed assault with intent to rob, and unlawful possession of a firearm, all in connection with Conneely’s Oct. 10, 2011, slaying in Dorchester.

Graham was also arraigned on two additional counts of armed assault with intent to rob and single counts of armed assault with intent to murder, assault and battery with a dangerous weapon, aggravated assault and battery with a dangerous weapon, and unlawful possession of a firearm in connection with the attempted robberies and non-fatal shootings of two other men on Oct. 30, also in Dorchester.

The Suffolk County Grand Jury returned the nine-count indictment on Dec. 20, Conley said, while Graham was held on $100,000 cash bail on a Juvenile Court complaint charging him with the Oct. 30 incident.

Assistant District Attorney Ian Polumbaum of Conley’s Homicide Unit told Clerk Magistrate Gary D. Wilson that Conneely, an Irish immigrant, was walking home to his Nahant Avenue apartment late on the night of Oct. 9.

“Neighbors reported hearing a loud ‘bang’ shortly after midnight,” Polumbaum said. “Conneely was found at 1:00 a.m. a few yards from his residence dead from a gunshot to the chest.”

Similarly, the victims of the Oct. 30 incident were walking home along Monsignor Lydon way when they were approached from behind by a person with a gun who ordered them to empty their pockets.

“They thought the weapon was fake and refused,” Polumbaum said. “The defendant opened fire, striking one victim four times from behind and the other victim in the arm. The first victim suffered life-threatening injuries.”

A break in the case and the connection between the two incidents was made when Boston Police compared the ballistics evidence in the Oct. 10 and Oct. 30 shootings. That comparison showed that the same gun had been used at both scenes, Polumbaum said.

Evidence developed by Boston Police detectives on the street and Suffolk prosecutors in the grand jury included video footage from surveillance cameras in the area. The investigation led to Graham and “other persons aware of his involvement” in the crimes, Polumbaum said.

Graham allegedly made statements to others that he had shot Conneely in the chest near Adams Street while trying to rob him. He also allegedly stated that he shot the two surviving victims after they resisted his robbery demands. Polumbaum said Graham’s statements were consistent with other evidence, including the surviving victims’ own accounts of the incident.

Graham is represented by attorney Randy Gioia. His trial has been scheduled for Jan. 7, 2013, and he is due back in court on March 6.