Zontre Mack will spend at least the next three decades of his life in prison for killing 15-year-old Curtis Ashford Jr. in Dorchester in 2022.

“Curtis Ashford’s family will mourn his loss forever and Zontre Mack will spend a large portion of his adult life away from his family, all because of a quick and deadly decision to pull the trigger on an illegal gun,” Suffolk DA Kevin Hayden said following the sentence. “There is nothing but tragedy and sadness in such a scenario.”

A Suffolk Superior Court jury in early October convicted Mack, 21, of Canton, of first-degree murder as well as unlawful possession of a firearm. On Wednesday, Judge Mary Ames sentenced Mack to life in prison with the possibility of parole after 30 years.

A co-conspirator, Dominick Gavin, 27, of Boston, pleaded guilty in August to being an accessory after the fact, for which he was sentenced to three to four years in prison.

The Supreme Judicial Court ruled earlier this year that no one under 21, who they termed “developing adults,” could be sentenced to life in prison without the possibility of parole. Mack was 18 at the time of the murder, according to the DA’s office, so he was offered parole after 30 years.

Ashford promised his mother he would be home by 8 p.m. when he left to go play basketball with friends at a playground in the Erie-Ellington neighborhood in Dorchester on July 27, 2022.

By 7:30 p.m., Mack and Gavin drove into the neighborhood and Mack drew a pistol. He fired 15 shots at the group of teens, including Ashford, who was struck in the back and would die from his injuries.

Ashford’s aunt, in a victim impact statement, told Mack in court that her brother, Curtis Ashford Sr., was killed in a similar way 16 years ago from someone just like you.

“Someone who didn’t know him, someone who had no regard for the lives of others and what their actions would do to the families of the ones they hurt,” she said.