Someone Called The Author-ities!

Riddle: Reuben, a renowned author, was found unconscious in his study on a Sunday morning. His agent, Kelly, called the police to report it. When the officers arrived, they questioned three suspects: Anna the nanny, Mike the cleaner, and Mark the driver. Anna said that she had driven Reuben's daughter to school. Mike said that he had a day off and spent it with his family. And Mark said that he had taken Reuben's car to the car wash. Who attacked Reuben?