From this point on, the killer started communicating via letters and greetings cards. Each of these messages was concluded with the crossed-circle design pictured above. Later it became known as the Zodiac signature. These letters as well as some coded messages were sent by the killer to different, well-known newspapers, including the San Francisco Examiner and the Vallejo Times Herald.
Many of these notes still haven't been decoded, and the murders have yet to be solved. See the coded messages at Environmental Graffiti. Link