Who Killed Mr. Olson?

“Who Killed Mr. Olson” can be played in two ways.  You can either print out the story without the answer or read the story and have the guests try to figure out who is the killer.  You could even divide the guests into teams and have them compete against each other.

The police were checking the office where Mr. Olson had been murdered. Outside, Mary, Mr. Olson’s faithful secretary, was a mass of nerves. Inspector Will was reviewing the facts:
Mr. Olson has been murdered. The murderer buried a stiletto through the back of the chair and right through his heart. The place was a mess, but nothing seemed to be lost. Some sheets on his desk had coffee stains, but Mr. Olson was not a coffee drinker. There was no coffee container of any kind in the office. There were a couple of gloves on the floor. Mary had in her visit book just three announcements:

Hari Colton (2:35)

Louis Williams (3:10)

Joseph Markis (3:45)

 Mary says that Joseph was the only one to serve himself a cup of coffee in a paper cup. The cup of coffee was found in Mary’s wastebasket. She claims that Joseph entered Mr. Olson’s office with the cup in his hands, and then he left it on her desk, so she took it and threw it away. The stiletto had no fingerprints at all, but the paper cup was all covered with one type of fingerprint: Joseph’s (they had a mobile fingerprint lab). Mr. Olson’s wristwatch was broken, probably due to the fight, and it showed the time at 3:50.

Inspector Will then talked to Mary, to tell her she was under arrest.
Why did he suspect her?

Answer

1. The watch could not have been broken during a fight. It was clear that there was not a fight because Mr. Olson was stabbed in the back through his chair. So, the time was a setup.
2. The mess and the coffee was a setup, because there was no fight and it would be a little difficult for Joseph to enter and put on some gloves to fight with a cup of coffee in his hands.
3. The last proof is that the cup of coffee only had one type of fingerprints. If Mary touched the cup (which she says she did), there should be some fingerprints of hers on it, unless she was wearing some gloves.