A gang member fired 10 shots into a crowd of students outside a Dorchester high school in June 2016, killing 17-year-old Raekwon Brown, who died carrying “a backpack full of school work,’’ a prosecutor said Thursday.

Assistant Suffolk District Attorney Thomas Flanagan laid out the harrowing scene for jurors during his opening statement in the trial of the alleged gunman, Jaden Waiters, 21, and two codefendants, Jonathan Aguasvivas, 25, and Benzy Bain, 26.

All three men are charged with first-degree murder and weapons charges stemming from the attack, which also wounded two other teens.

Flanagan told jurors in Suffolk Superior Court that video surveillance captured the rotund Waiters, sometimes called “Fat Boy’’ in his Dorchester neighborhood, as he opened fire in the area of Washington and Normandy streets outside Jeremiah E. Burke High School around 1 p.m. on June 8, 2016. Students had exited the school after a fire alarm went off inside.

“This individual, Mr. Waiters . . . didn’t hesitate,’’ Flanagan said.

Robert Sheketoff, a lawyer for Waiters, countered that the video footage of the shooter doesn’t clearly depict his client. If jurors see Waiters in it, Sheketoff told them, “You have better eyes than everybody else.’’

Flanagan said Waiters fled on foot to Supple Road, where Aguasvivas and Bain were waiting in a van rented by a woman who listed Aguasvivas as an authorized driver of the vehicle.

And, Flanagan said, a man on Supple Road noticed the van idling before he heard shots and later saw a “heavyset young black male’’ rush into the vehicle, which sped away.

“These three individuals acted together,’’ Flanagan said.

Sheketoff tried to discredit the expected testimony from the man on Supple Road, stating police “never bothered’’ to show the man photographs to see whether he could identify Waiters as the person who got in the van.

Instead, Sheketoff said, authorities are relying “on some sort of affiliation, that [Waiters is] some sort of gang member, therefore it must be him.’’

With relatives of Brown and the defendants looking on, Flanagan discussed additional evidence that he said tied all three suspects to the killing.

That evidence includes GPS data from a bracelet Bain wore at the time while on probation, cell tower data, and video surveillance that tracked the trio’s whereabouts before and after the murder.

In addition, Flanagan said, police recovered 9mm ammunition from the homes of Aguasvivas and Waiters that was “consistent with’’ ballistics at the crime scene. No murder weapon was found.

Though he identified all three defendants as members of the same gang, Flanagan did not provide jurors with a motive for the killing.

Speaking from the bench, Judge Jeffrey Locke said before jurors entered the courtroom that prosecutors believe the shooting could be linked to a prior dispute that Aguasvivas had with a member of a rival gang who was seen in the area outside the school before the murder.

Presenting that theory to the jury will be “dependent on the Commonwealth’s ability to produce [the gang rival] as a witness,’’ Locke said.

Later during opening statements, Bain’s lawyer, James Greenberg, said his client “didn’t kill anybody’’ and “didn’t help to kill anybody.’’ Police found no evidence connecting Bain to the murder when they searched his home and “his nightmare began,’’ Greenberg said.

Brian Kelley, a lawyer for Aguasvivas, said no one identified his client as a participant in the murder. He said the 9mm ammunition found at Aguasvivas’s home is “very common.’’

“It’s like a Michelin tire,’’ Kelly said, adding that Brown had a physical altercation with another youth earlier on the day of the shooting.

“This was a terrible tragedy, but it’s not one my client was involved in,’’ Kelley said.

Brown’s sister, Latasha Allen, 33, later took the stand as the first trial witness.

She described her sibling as a “regular kid trying to achieve his goals in life’’ and fought back tears when Flanagan displayed a photo of Brown dressed in a tuxedo for a wedding and asked her to identify him.

“My baby brother,’’ she said.

Travis Andersen can be reached at travis.andersen@globe.com.