Can you solve these?

I just received this interesting mystery from hubby few days ago. Unfortunately my IQ it was not good enough to answer this mystery. So, guys if your iq is good than mine don't hesitate to answer this.

The Mystery

A man was found murdered Sunday morning. His wife immediately called the police. The police questioned the wife and staff and got these answers: The wife said she was sleeping. The cook was preparing breakfast. The gardener was gathering vegetables. The maid was getting the mail. The butler was polishing shoes in the pantry. The police instantly arrested the murderer. Who did it and how did they know?