The official told CNN that the response began with a 911 call from the clinic, which was given the highest priority.
The fire department were first to arrive at the clinic, five minutes after the call.
Medical staff were already trying to give Rivers CPR using a defibrillator and a breathing tube.
The firefighters then took over the attempts to revive her.
Two minutes after the firefighters came, an ambulance arrived at the clinic, whose paramedics helped with the CPR.
Almost immediately after that, another Fire Department vehicle arrived with medical aid, making a total of more than 10 people fighting to save her.
Minute by minute: How rescuers battled for Joan
9:40 a.m, August 27: Yorkville Clinic called 911 with reports of a cardiac arrest.
9:45 a.m: The New York Fire Department response team arrives to find CPR underway, and takes over.
9:47 am: An ambulance from Mount Sinai Hospital, New York, arrives and joins in.
9.48am: A second NYFD unit with Emergency Medical Technicians, along with a supervisor, arrive at the clinic.
9:50 am: By this point the number of rescuers totals ten people.
- Daily Mail