The Missing Diamond

A famous diamond was stolen from a museum during the night. The police suspected four people: the curator, the security guard, the janitor, and the electrician. They interrogated each of them and got the following statements:
- The curator said: "I was at home all night, working on the catalog for the next exhibition."
- The security guard said: "I was on duty until midnight, then I went to the nearby diner for a coffee. I have a receipt to prove it."
- The janitor said: "I was cleaning the museum until 11 pm, then I locked up and went home. I have the keys to the museum with me."
- The electrician said: "I was fixing some wiring in the basement until 10 pm, then I left. I have a work order to prove it."
The police checked the alibis and found that they were all true. However, they also found a clue at the crime scene: a pair of pliers. Who stole the diamond and how?

Solution: The electrician stole the diamond. He used the pliers to cut the wire of the alarm system in the basement, then he went upstairs and took the diamond. He left the pliers behind by mistake. He lied about leaving at 10 pm, he actually stayed until after midnight, when the security guard left. He then used his work order to pretend that he was there for a legitimate reason.