Who was this in my SINGLE PLAYER GAME

I just recently finished the main story in ready or not. I loaded up for the first time a SINGLE PLAYER quick match game. I set it to hard, and played the mission “thank you, come again”. The first thing I did was told my team to hold, because I wanted see what my team would do, if I just started to kill civilians without clicking the “kill me” command. After getting almost everyone talk called me an active threat and my team started to move on the gas station without me clicking the “kill me” option. They started securing evidence and I noticed a 6th AI in my game. I forget the name but it was something starting with an N. He came up to me and killed me, the weird thing was, he was dressed fully in the 90’s swat uniform you get once completed the game, but everyone else was in the default uniform. It was also single player so I have no clue how a 6th person could have joined. And after I had died and clicked spectate, I could flip through all teammates POV except the unknown person. Does anyone know how this happened or who this was?