Tina was assaulted outside of the room with something that would render her unconscious, then killed in the room where she was dragged into, and then locked in with a key.
Tina was killed by someone throwing a gas bomb or some sort of poison into the room so Tina would suffocate/get poisoned and die, right before she locked herself in.
Someone shot Tina with one of the silenced pistols through the keyhole/a wall/a window/another opening/the open doorway before she was able to close and lock the door behind her, and she bled out.
Tina is feigning her death, and upon us/me leaving the area, she was able to leave the vicinity unharmed to help cover up the mysteries.