I don't think anyone did. I think "any detectives out there" is a clue.
4 has ripped shirt. A piece is under victim. His holster is showing. He had an altercation with victim. That's to easy though.
3 she has an empty purse on table with lines on side of plate. This appears to be a communication device. Lines depict sound. Her head wear was probably hidden in purse. She's undercover. My guess she is wearing a wire.
2 not sure put he is looking at 4 but if he was suspicious of him he probably wouldnt still be there. 1 is talking to 3. He warns her of something.

My guess is no killer. They are undercover and suspect was at the table behind 2. Its missing a knife and and table looks messy. The victim and the chef was up to something. Why is the chef serving? Guess victim is the waitresses. She saw 4 gun and tried to take it. 3 helped and his her in restroom to also catch accomplish.

The open door goes to kitchen. The chef just came out.

Their may be more to .