Cassano said Friday that a dispatcher received the emergency call after Tuesday's crash on the Upper West Side, but it took four minutes before someone spotted the call.
From then, it took four more minutes for an ambulance to reach the scene.
Cassano says glitches in the city's new 911 system were not to blame for the delay, but he added it's not clear whether a faster response would have helped Ariel Russo.
Police say an unlicensed 17-year-old driver, Franklin Reyes, struck the girl and her grandmother at 97th Street and Amsterdam Avenue while fleeing police. Russo was pronounced dead at the scene. Her grandmother was taken to the hospital in critical condition with leg and back injuries, but was later upgraded to stable.
The teen has been arrested on charges of vehicular manslaughter and is being held without bail.