Some towns and counties have cancer clusters. Others have high rates of traffic fatalities or foreclosures. In New York, Long Island has been grappling with its own disturbing demographic: Three serial killers who targeted the same type of victims have operated in the same tidy suburbs in the span of 22 years.

Since 1989, when the first victim was killed, the three have been active in Long Island’s two counties — Suffolk and Nassau — in apparently unrelated cases. Joel Rifkin, a 34-year-old unemployed landscaper from East Meadow, confessed to killing 17 women he said were prostitutes following his arrest in 1993. Robert Shulman, a 42-year-old postal worker from Hicksville, was convicted of killing five prostitutes after he was arrested in 1996. The third killer has yet to be apprehended, but the police know that he or she exists: the bodies of eight people — at least four of them female prostitutes — have been found in the thick brush near a Suffolk County beach since December.

It is a phenomenon that, though rare, has occurred elsewhere. Beginning in the early 1980s, at least five serial killers terrorized the South Los Angeles area, including Lonnie Franklin Jr., whom the police accused of being the so-called Grim Sleeper (he took a hiatus from 1988 to 2002).

Serial-killer experts and others offered a host of theories for the cluster phenomenon. Several said it was a fluke of geography. Still others said it was, in a sense, an illusion: Other cities and regions could have just as many or even more serial killers in their midst, but their patterns have not yet been detected by the police.