Only Law enforcement Contact Teams operate within the Hot Zone. No fire, EMS, or unarmed personnel enter the Hot Zone.
There are some agencies who do send their Rescue Task Force teams (with security) into a known Hot Zone as part of their standard operating procedure. While rare, entry into a known Hot Zone by Rescue Task Force teams is a local policy decision.
It's important to note zones (e.g., hot, warm, cold) are often depicted as a clean geographic area such as a square or circle. In practice, this is rarely the case. In reality, zones tend to be shaped like arbitrary blobs of completely irregular shapes. And, the shape of each zone changes over time (like an amoeba) as the active shooter incident unfolds. Over time, the Hot Zone should continue to shrink until only a Warm Zone remains. When clearing begins, the Warm Zone will shrink as the Cold Zone grows until the scene is declared safe.