Old checks, past telephone bills, and post office address location services may provide leads. The Social Security Administration will forward letters to persons for whom they have addresses, and the Salvation Army may be able to provide information on homeless persons. Other sources of information include college alumni associations and armed forces personnel records centers. Professional organizations, business directories, old neighbors, historical and genealogical societies, and libraries also may provide helpful information. In addition, a number of public and private missing persons and search organizations are available, including those specializing in missing parents, children, and friends. The National Crime Information Center maintains a missing person and an unidentified person file. Finally, magazine articles and books can provide additional, more detailed information on strategies and techniques for locating missing persons.