What do I need to do here?

Hint 1 of 17:

Quite a lot!

Hint 2 of 17:

Talk to everyone.

Hint 3 of 17:

Look closely at the slain police officer's body.

Hint 4 of 17:

Take notes

Hint 5 of 17:

(use the notebook) on the body and

Hint 6 of 17:

the cigarette beside it.

Hint 7 of 17:

Take notes on the graffiti on the wall.