Two accused gang members teens were arraigned Thursday on charges they murdered 16-year-old Jaivon Blake and wounded Blake's younger friend in a fusillade of bullets aimed at avenging alleged wrongs by rival gangs.

Sheldon Mattis and Nyasani Watt were both arrested Thursday. Assistant Suffolk County District Attorney Mark Hallal said that Watt was packing a .38-caliber gun loaded with ten bullets, one in the chamber, when arrested outside his home.

Both men were ordered held without bail. Both exercised their right to stay behind a door rather than face Blake's family and friends and reporters from the prisoner dock in a Dorchester District Court courtroom as they were formally accused of a variety of charges including murder, assault with intent to murder, assault and battery with a dangerous weapon and illegal possession of a loaded firearm and ammunition.

Hallal said Mattis and Watt are members of a gang calling itself Flatline, which had been under attack by other gangs in the area. On the afternoon of Sept. 25, Hallal said, members were hanging out in front of 6 Levant St., plotting revenge plans. Mattis got on a bicycle and rode up to Geneva Avenue to look for anybody who belonged to those gangs.

At the corner, Hallal said, he ran into the unidentified 14-year-old friend of Blake's, himself hanging out in front of a convenience store there. He asked Mattis to buy him some rolling paper. Mattis agreed, went inside, came out with the rolling paper and gave it to the kid, Hallals said. Then he rode back to 6 Levant and told Mattis he thought the kid belonged to the Geneva Avenue Gang, one of the groups that had been attacking his pals.

As the boy walked toward the Geneva Avenue Walgreens to meet Blake, Mattis and Watt agreed to take him out, Hallal said. Watt got on a bike, strapped a .40-caliber gun around his waist. Mattis helped smooth out his shirt, then "patted him on his back and sent him on his way," Hallal said.

On Geneva, Watt took out the gun and fired at least six times shortly before 3:30 p.m. The younger teen got hit in the neck, back and shoulder, Hallal said. Blake fell, mortally wounded. Watt pedaled furiously down Olney Street and disappeared as police arrived, Hallal said. Officers started CPR and managed to revive him, but he died of his injuries a few hours later at Boston Medical Center, Hallal said. The younger teen survived.

Hallal said Watt will be tried as an armed career criminal because of past offenses, which include three counts of assault and battery on a police officer - he was on probation for that out of West Roxbury District court. Mattis received a suspended juvenile sentence for larceny over $200 in 2010, Hallal said.