Problems with radio communication caused commanders to lose contact with many of the firefighters who went into the buildings. The repeater system in the World Trade Center, which was required for portable radio signals to transmit reliably, was malfunctioning after the impact of the planes. As a result, firefighters were unable to report to commanders on their progress, and were unable to hear evacuation orders. Also, many off-duty firefighters arrived to help, without their radios. FDNY commanders lacked communication with the NYPD, who had helicopters at the scene, or with Emergency Medical Services (EMS) dispatchers. The firefighters on the scene also did not have access to television reports or other outside information, which could help in assessing the situation. When the South Tower collapsed at 9:59 a.m., firefighters in the North Tower were not aware of exactly what had happened. The battalion chief in the North Tower lobby immediately issued an order over the radio for firefighters in the tower to evacuate, but many did not hear the order, due to the faulty radios. Because of this, 343 firefighters died in the collapse of the towers.