Can YOU solve this mystery?

Ready to be a detective? YOU solve this one! I’ll send you one of my short stories if you can identify the killer. If no one noodles it through—aw, say it isn’t so—then I’ll have the answer in my next post. Good luck, fellow sleuths!

The Mystery of the Locked Office

The Setup: Detective Max Harper is called to investigate a suspicious death at the Acme Publishing Company. The victim, Mr. Edgar Brooks, the senior editor, was found dead in his locked office, slumped over his desk, with a single stab wound to his back. There was no sign of a struggle, and the office key was still in his pocket. The only other entrance, a window, was locked from the inside.

The Clues:

1. Mr. Brooks’ assistant, Ms. Riley, said she last saw him alive when she brought him coffee at 8:30 AM.

2. A cleaner, Mr. Watkins, claims he heard muffled arguing from the office around 8:45 AM but assumed it was just a heated phone call.

3. A paper cutter, with its blade missing, was found in the storage closet down the hall.

4. A single fingerprint on the office door handle matches Ms. Riley’s. She claims she only touched it earlier that morning when delivering mail.

5. The coffee cup on Mr. Brooks’ desk is half-empty but still warm.

The Question: How was Mr. Brooks murdered, and who is the most likely culprit?

Hint: Pay attention to the timeline, the office layout, and the missing blade.