Carter Brown Mysteries - Kudos for Khan

Joe Khan, a private detective, was talking to his secretary when his office phone rang.  It was from a woman named Diane who needed to see Joe.  Later that morning, Diane walked into Joe's office.  She told him that she needed help to find her sister, Helen.  Helen married an American serviceman and moved with him to the states.  About three months later, Helen's husband was killed in an auto accident. Diane has not been able to find her since that time. Diane called the Missing Persons Division of the police, but they forwarded her to Lieutenant Harding in the Homicide Bureau, Joe accepts the job, and goes to see Harding. Joe asked him why Homicide is interested in a missing persons case. Harding did not offer any information, other than to not become involved in this situation and tell Diane that he cannot help her.