Detective Case #1: Who is the Killer?

A billionaire was found murdered in his study on a rainy Sunday afternoon. The detective questioned the three suspects who were in the house: 1. The Cook: He said he was preparing dinner. 2. The Butler: He said he was polishing the silver in the dining room. 3. The Gardener: He said he was watering the flowers in the garden. Who is the killer? (Answer in one word: cook, butler, or gardener)