Also, I'm thinking the BANG could have been
The murderer probably got in
And the burned documents probably were
This should leave both hints satisfied and the mystery... hopefully solved!
Edit: this answer does not account for the locked door, so I consider this to not be a great solution. (It would be cheating to add "and the attacker had a key", since that's against the spirit of the question design: the door was not opened from the outside.)
I have several theories;
9:58 AM: Bob goes to visit Adam - 10:12 AM: Bob leaves Adam's apartment and goes to his own.
10:45 AM: Nicole visits Adam. - 11:02 AM: Nicole leaves Adam's and visits Charlie's.
11:02 AM: Nicole leaves Adam's and visits Charlie's. - 12:04 AM: Nicole leaves Charlie's. Her shirt is now on backwards.
Ok now this is crazy and 100% not the intended answer however I want to bring up the possibility that
Well, how is this even possible and why would they do this?
And here is the punchline
So the murderer most likely pulled off their stunt perfectly because the crime scene implies that