Murder Riddle (All ages, Medium)

Question: A rich man and his son live in an eight-story bright blue circular house together. It has green windows and neon yellow curtains, all with dazzling pink carpeted floors. One afternoon, the son is killed. The father is furious and sets out to find the murder. He collects alibis to send to the NYPD. The Maid was out Holiday shopping with the father. The Father was out Holiday shopping with the maid. The cook was preparing dinner. The butler was cleaning the corners. The gardener was setting up some flowers on one of the window sills. The nanny was in the hospital. The NYPD arrested the criminal as soon as they heard all the alibis. Who was it and how did they know?