The last time Latrina Fomby-­Davis saw her 14-year-old son alive, he was headed out the door of their Dorchester home. He hopped on the back of his brother’s scooter, and the two boys rode around their neighborhood, stopping for haircuts and to hang out with friends.

Soon after he left, Nicholas was shot twice, once in the leg and once in the chest, allegedly by two slight teenagers described Friday by a Suffolk prosecutor as a “killing team.”

Latrina Fomby-Davis, one of four witnesses who testified on the first day of testimony in the trial of the accused killers, described her final moments with her lifeless son at the hospital that night in May 2010.

“I sat there and cried with my baby for about 15 minutes,” she said, her voice filled with grief.

The boy was killed on the corner of Olney and Bowdoin streets, an innocent victim of “petty, ridiculous rivalries” in the neighborhood that for years had festered in the neighborhood, Assistant District Attorney Patrick Haggan said during his opening statement.

He told the jury of nine men and seven women that it may never be clear why Joshua Fernandes, 18, and Crisostomo Lopes, 22, killed a boy they did not know. Both men have pleaded not guilty to murder charges.

Nicholas Fomby-Davis was riding alone on his brother’s scooter when Lopes pulled him off, held him, and “encouraged” Fernandes to fire, Haggan told jurors.

“I suspect at the end of the case, it won’t make any sense to you,” Haggan said.

But he said that surveillance video, witnesses includ­ing an off-duty police officer who saw the shooting, and forensic evidence would prove they were respon­sible for his death.

Witnesses, Haggan said, would describe how ­Fernandes, who was 16 at the time, smiled as officers handcuffed him minutes after the shooting, while Lopes defiantly shouted “Homes Ave. on the block!” during his arrest. Homes Avenue is a Dorchester street with a gang associated with it, prosecutors said, alleging that Lopes is a member.

Fernandes, dressed in a light blue-green dress shirt and tie, and Lopes, who wore a dark suit, sat silently during the proceedings, their faces expressionless.

Fernandes is also charged with unlawful gun possession.

His lawyer, Rosemary ­Scapicchio, said during her opening statement that police focused on the wrong person.

She said witnesses described the shooter as a male with corn rows, who wore a white shirt and basketball shorts. Her client, she said, was wearing dark jeans shorts.

Another young man, a witness for the prosecution, fits the description of the shooter better, she told the jury. That young man was seen leaving the scene of the shooting, then rushing back with two other men to talk to Fernandes as police placed handcuffs on him, Scapicchio said.

“I suggest to you that they wanted to threaten Joshua ­Fernandes,” Scapicchio said.

Haggan described how ­Fernandes later told police that he was alone at the scene and could not remember what happened because he “blacked out.”

Scapicchio said her client made that “ridiculous” statement to police during his interrogation because he feared those men at the scene.

“He was 16 years old,” Scapicchio said. "He had been threatened, and he was afraid.”

Christopher Belezos, Lopes’s lawyer, said that for many reasons, the prosecution will be unable to prove Lopes played a role in the murder.

“One, he never shot anyone,” Belezos said. “Two, he never had a gun. Three, he never ­arranged for anyone to have a gun.”

Belezos and Scapicchio both said in their opening statements that the surveillance footage would actually help their case.

The footage, they said, shows three males in white T-shirts in the area at the time of the shooting, not two, as prosecution witnesses are expected to say.

“The video surveillance has no agenda,” Belezos said. “The video surveillance has no emotions. The video surveillance has no bias.”

The trial is expected to last two to three weeks. During testimony Friday, Scapicchio asked two prosecution witnesses who were at the scene if they had been showed an array of photos of potential suspects. They said they had not.

She and Belezos also aggressively questioned a police witness, one of the first to arrive at the scene, about inconsistencies between his testimony and the report he wrote that night.

Neither defense attorney cross-examined Fomby-Davis’s mother, who described her son as a drummer at his church, a boy determined to stay far from the tension that could sometimes plague his neighborhood.

“He just liked to play video games,” she said, speaking slowly. “The kids he played with could find a stick and a rock and have fun with it all day.”

After her testimony, Latrina Fomby-Davis slowly walked back to the gallery to sit next to her husband, Nathaniel, who put his arm around her.

The defendants did not look at her.