Yasin Harvey, 16, who was not a member of either gang, was shot in the head, left arm and twice in the chest while standing in front of a corner store at 62nd and Callowhill.
Fleming was driving, Brown fired the .38-caliber weapon while Kareem Terry, a third gang member who owned the car, sat in the back seat, O'Neill said.
"They were out hunting. They were so reckless in what they did. They killed an innocent kid," said O'Neill, who asked the jury to convict Brown and Fleming of first-degree murder.
They are also charged with second-degree murder, third-degree murder, conspiracy and carrying a firearm without a license.
Terry, 21, will testify against Brown and Fleming, O'Neill told the jury. He was not charged in Harvey's death.
Attorney Jack McMahon, for Brown, and attorney David Mischak, for Fleming, both told the jury of seven women and five men that Terry was the gunman who killed Harvey.
Three days after the slaying, police stopped Terry in the car from which the gun was fired. After four days in police custody he concocted a statement implicating Brown and Fleming, the defense lawyers said.
McMahon suggested that Terry was rewarded for lying about his fellow gang members by being granted immunity from prosecution for agreeing to testify against them, and by getting probation in November 2012 for a gun charge that predated Harvey's slaying.
He bristled that Terry - who has since been arrested for robbery and is behind bars - is a "scammer" and "manipulator" who is playing the system.
"He's making a mockery out of justice, making a fool out of the homicide detectives. Hopefully, not of you," McMahon told the jury.
Mischak told the jurors that they will hear a phone call between his jailed client Fleming and Terry during which Terry confesses to killing Harvey by saying, "I know I did this." During that call, Mischak said, Fleming will be heard telling Terry, "I didn't tell you to do it."
Harvey's mother, Monique Harvey, in 2012 told the Daily News that they had recently moved here from Atlantic City to escape violence in the shore town. Yasin Harvey would have started 10th grade that fall at University City High, she said.
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