Mayor Cory Booker, who is running for the U.S. Senate, was on patrol with DeMaio when the killing occurred, The Star-Ledger of Newark reported. Booker announced new anti-violence initiatives Wednesday.
The boy was shot in a courtyard at the Riverview Terrace, a housing complex where he lived, DeMaio said.
Newark is not the only New Jersey city to have had a violent summer. Trenton broke its annual record for homicides with 32 by the end of August with four months left in 2013.
DeMaio said most of the recent homicides have been in the same district in South Newark. He said about half are believed to be related to drug trafficking.
"It's just kind of a random spark-up," he said. "There's been no change in the deployment strategy we used all summer."
Booker announced Operation Blue Zone, which will focus police resources on drug activity, a spokesman, James Allen, said.
The mayor is the Democratic candidate in October's special election to fill the seat held by the late Frank Lautenberg, D-N.J. Booker is favored to win in a state that has not elected a Republican to the Senate since 1972.