“Let’s call it what it is,” the prosecutor said from a lectern facing 16 jurors chosen from hundreds for their fairness and impartiality. “It’s the execution of a 16-year-old boy.”

Assistant District Attorney John Pappas was referring to the Dec. 1, 2008, shooting of DeAndre Barboza near the intersection of Lyndhurst and Washington streets. Barboza sustained injuries that claimed his life two days later.

On trial for the Mattapan teen’s homicide is PATRICK GRIER (D.O.B. 7/11/88) of Dorchester, who allegedly fired several shots at Barboza at 9:08 a.m. amid the bustle of a busy Codman Square street corner and then fled on foot.

“He armed himself with that Harrison and Richards .22 caliber firearm,” Pappas said. “He made the decision to conceal it on his person. He made the decision to draw that weapon. He made the conscious decision to point that weapon and shoot it. This was not an accident.”

Acting on a description provided by witnesses at the scene who said the gunman ran down Aspinwall Road, responding Boston Police apprehended Grier and his longtime friend, a girl then 16, moments later on Colonial Avenue.

The girl was carrying a .22 caliber revolver later determined to have been the murder weapon. She testified at trial that Grier had given her the gun after the shooting and told her to “take this.” She has been charged as an accessory after the fact and is being prosecuted separately.

Retracing his path of flight, Boston Police recovered a distinctive, multicolored baseball hat on Aspinwall Road. The hat carried Grier’s DNA.

“You saw how unique that hat was,” Pappas said. “Doesn’t that look like the kind of hat someone might remember? Isn’t that the kind of hat that, when you murder someone in broad daylight, you might shed?”

By tossing the hat and giving the murder weapon to his teenage friend, Pappas said, Grier was “getting rid of an identifiable article of clothing and the gun.”

When it was later tested, Pappas said, Grier’s jacket was found to have gunshot residue on the cuff.

“One man, one gun,” Pappas said. “First-degree murder, nothing less.”

At the end of Pappas’ argument, Suffolk Superior Court Judge Judith Fabricant began instructing jurors on the relevant law. If jurors do not reach a unanimous verdict today, they will return to courtroom 907 on Friday morning. There will be no deliberations tomorrow because of the Bunker Hill Day holiday.

Catherine Yuan is the district attorney’s victim-witness advocate on the case. Grier is represented by attorney David Apfel.