Speak with Justinian. He wants you to go to the proceedings of the trial against the accused. When ready, he'll port you there.
Once there, speak with the Seventh Hammer, then the Defendant, defender, the Prosecutor, view the evidence, and question the witnesses. This step involves no fighting, just a lot of clicking and reading. When done, speak to the Seventh Hammer and give your verdict, guilty or not.
Choose not guilty. The evidence was incomplete (the note found was torn and words missing), and couldn't be considered conclusive.
Head back to the door and out to Justinian. Speak with him to receive your cloak.