This is for my forensic class so please help<3 thank you

A man is found murdered on a Sunday morning. His wife calls the police, who question the wife and the staff, and are given the following alibis:

-The wife says she was sleeping
-The butler said he was cleaning the closet
-The gardener was declared he was picking vegetables
-The maid explained she was getting the mail
-The cook said she was preparing breakfast

Immediately, the police arrest the murderer. Who did it and how did the police know?