if im not mistaken any sort of police/swat that enters someones house warrent or not the second there threw the door they have to identify themselves.
this would be because 1. so they establish its not someone breaking into there house and trying to steel something 2. if you shoot a cop chance are your going away for awhile 3. legal reasons
dont get me wrong i could be incorrect but any swat show ive ever watched on tv, i always hear them say something along the lines of “swat, everyone down.” or something like that.