As noted, Graysmith also had an explanation for how the Zodiac might have disguised his printing, but clearly the letters have not been linked to anyone for certain. The fact that the Zodiac stopped killing has been the focus of a good deal of speculation as well, including that he could be dead, that he might be in prison for some other crime, or that for some unknown reason he just stopped. No one really knows the answer to this question, either. A killer in New York later used the name Zodiac and even claimed to be the Zodiac, but that has been discounted by most observers. Earlier killings in Riverside and elsewhere were alter attributed to the Zodiac as well, though again, no one is certain if this connection is real or not. Law enforcement has become better equipped over the past thirty years to cope with this sort of crime, with the aforementioned use of profiles, with DNA as a new tool, and with the creation of units dedicated specifically to this type of crime. Even so, numerous instances can be cited where progress was stymied just as much as it was for the Zodiac case for many years before the killer was found. The Green River killer in Washington state is one such...

1). Another case is that of the so-called BTK killer in Kansas, who operated from 1974 to 1991 and then again in 2004 until Dennis Rader was caught. Such crimes can be virtually impossible to stop and very difficult to solve, and the Zodiac case is a clear example. Graysmith details how the police and others tried to solve the crimes and how they failed, though the crimes did stop for reasons unknown.
Works Cited

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, Quick Reference Guide (4th ed.). Washington, DC: American Psychiatric Association, 1994.

Egger Steven a. Serial Murder: An Elusive Phenomenon. New York: Praeger, 1990.

Graysmith, Robert. Zodiac. New York: Berkley, 2007.

Green River Killings." The Seattle Times (19 Nov 2004). April 1, 2007. http://seattletimes.nwsource.com/html/greenriverkillings/.

Leung, Julietta. The personality profile of a serial killer (2004). Retrieved April 1, 2007 at…