A Brain Teaser

Sometimes Mr. Harbin finds us tired and lazy, so he gives us a brain teaser just to make our brains work better. One of the riddles was: The police once received a call from an unknown number to report that a murderer named John, whom they were looking for was in a certain house. So, they went and forced open the door. There, they found four people playing cards: a lawyer, a teacher, a shop assistant, and a fireman. Without a word, they arrested the fireman.

Why did they arrest the fireman?

By Amina, Farah, and Marwa