The letter is an important piece of evidence in Broken Minds. It reads:
To Noa: Your parents are dead. Your parents are dead! They’re dead. DEAD DEAD DEAD. I think you know why. So you’ll be an orphan. Everybody likes orphans. And now... You’re one too. — The Orphan.
From this letter, the detectives are able to put a name to the killer. It is also the reason Noa decides to hurry back to her apartment, dragging Ume along.