The official spoke on condition of anonymity because the arrests were to be announced at an afternoon news conference. The official would not provide further details.
Aaron Shannon Jr. was flexing the muscles of his Spider-Man costume and posing for photographs in the backyard with his uncle, grandfather and cousin Sunday afternoon when two men in an alley opened fire on the group, police and relatives have said.
The boy, who was getting ready to go to a party, was shot in the back of the head and died at a hospital the next day.
The home is located in disputed gang territory and the attackers mistakenly believed that members of a rival gang lived there, police have said.
The city had offered a $75,000 reward for information leading to an arrest.