What made the police first believe that robbery was the motive for these murders, and what evidence was there to refute that idea?

Police discover a gold wristwatch tucked into Nancy's shoe, indicating that she might have been trying to hide her valuables from the intruders. Police also note that Kenyon's Zenith radio is missing. Robbery was ruled out as a motive because Herb Clutter was known for not keeping any money in the house.