A photo of a 10-year-old girl smiling for the camera. An x-ray of her tiny skull, peppered with shotgun pellets. The slain child’s head on a coroner’s slab.

One by one, the top prosecuting attorney in Suffolk County presented those images of Trina Persad, shot in the face with a sawed-off shotgun on the evening of June 29, 2002, to the Superior Court jury now weighing the fate of her alleged killer, JOSEPH COUSIN (D.O.B. 8/9/84).

“How,” asked First Assistant District Attorney Josh Wall, “does that happen? How does a little girl in our city go from life to death in a matter of seconds? How does a walk down Blue Hill Avenue with Auntie Kathleen become a death march?”

After two full weeks of testimony in Cousin’s trial, Wall approached the jury box for the last time. He reminded jurors of Persad’s final moments and how they were spent in a game of counting cars on a busy Boston street.

“How do we go from a child’s game of counting cars to the forensic task of counting pellets in a child’s brain?” Wall asked. “It happens because of gang warfare, gang rivalry, and gang hatred. Back and forth, back and forth, worse and worse.”

It happened, Wall said, in the context of a longstanding rivalry between two notorious Boston street gangs – the Big Head Boyz, associated with areas of Brunswick, Creston, and Fayston streets, and MIC, named for its turf on Magnolia Street, Intervale Street, and Columbia Road.

“To Joseph Cousin, the MIC is family,” Wall said, recalling that the rivalry between the two groups had picked up during the summer Persad was murdered and noting that the Big Head Boyz had fired on MIC associates the night before she was slain.

Wall recounted the testimony of three former gang members who testified despite the universal gang code against “snitching.” One was a member of the Big Head Boyz who agreed to identify Cousin as the gunman because the victim, at 10 years old, was completely uninvolved in the gang feud.

“She didn’t choose this life,” Wall said, paraphrasing his testimony. “Her family didn’t choose this life. I did choose this life. I wouldn’t tell if it was one of my people.”

A second former Big Head Boyz associate also identified Cousin as the man who shouted at him from a stolen car, then pointed and fired the shotgun at him and his associates at Jermaine Goffigan Park.

“It wasn’t a sneak attack,” Wall said. “He wanted to be seen. He yelled at them …. There was a very good opportunity for each of these people to see Joseph Cousin because he wanted them to know it was him.”

Wall recalled the testimony of a former MIC associate who gave a detailed statement to Boston Police homicide detectives after being told to “tell the truth” by his mother. That witness described the plan to steal a car, obtain a gun, and shoot members of the Big Head Boyz, and he described the execution of their “mission” on the evening of June 29, 2002.

That witness’s descriptions, Wall said, corroborate the statements of other gang members, the testimony of civilian witnesses, the findings of Boston Police detectives, and the forensic evidence developed by criminalists.

Wall laid out the deadly sequence of events one last time, from obtaining the stolen car to the final confrontation at Goffigan Park, where Cousin and his associates exchange words with the Big Head Boyz from inside the vehicle.

Cousin’s associates drove away but rolled back moments later.

“Joseph Cousin puts the window down again,” Wall said. “And the gun Joseph Cousin fired? The child was looking right at it. He fires at the Big Head Boyz and the little girl gets hit.”

Wall held aloft the sawed-off shotgun that killed Persad.

“When you think about deliberate premeditation, think about Joseph Cousin going to pick this up,” the prosecutor said before pumping the grip as a gunman would to force a shell into the weapon’s chamber. “Think about that when you think about deliberate premeditation.”

Wall laid the firearm down and asked, “Who was willing to fire a shotgun into a park and kill a little girl? Ladies and gentlemen, based on the evidence, you can conclude that the killer is here. The killer is in this courtroom.”

After closing arguments, Judge Nancy Staffier Holtz instructed jurors on the relevant law and they began their deliberations. They retired at about 4:30 and will resume tomorrow morning.