The Knox-Box Rapid Entry System is a small safe mounted on the exterior wall of a residential or commercial building that contains keys for first responders to gain access in the event of an emergency. The first responders have a master key.
Local emergency services have lists of area Knox-Box codes for crews to use with the master key. This prevents firefighters and EMTs from needing to use force to enter a building.
There is some controversy surrounding Knox-Boxes. If a thief gained access to the area's codes, all local businesses would be at risk for burglary. Additionally, a key can be made through disassembling a new Knox-Box.