A study of nearly 40 years of terrorist attacks on U.S. soil reveals that nearly a third took place in just five major urban "hot spots": New York, Los Angeles, Miami, San Francisco and Washington, D.C.

The report, by researchers for the National Consortium for the Study of Terrorism and Responses to Terrorism, found those cities, along with Chicago and Seattle, experienced the most attacks motivated by causes, ranging from left-wing and right-wing agendas to religious and nationalist ones, as well as some single-issue rationales. Their findings were based on information in the consortium's Global Terrorism Database.

Manhattan saw the most attacks, with 343. Los Angeles was next with 156, Miami-Dade County had 103, San Francisco had 99 and Washington had 79, according to the report. In all, the researchers identified 65 of 3,143 U.S. counties as hot spots.

Although big cities have remained the most frequent targets of terrorist activities over the decades, smaller, more rural counties such as Arizona's Maricopa County -- which includes Phoenix -- have emerged as hot spots in recent years as domestic terrorism in those areas has increased, the report says.

Researchers defined a "hot spot" as any county hit by terrorism more than six times from 1970 to 2008, the most recent year studied

The full report is available here: http://start.umd.edu/start/publications/research_briefs/LaFree_Bersani_HotSpotsOfUSTerrorism.pdf