International Terrorism in the US
Alex Tabarrok reports on foreign linked terrorism in the United States. The RAND data base of international terrorism counts 567 acts of international terrorism in the US between 1968 and 2009. 140 of these, or about 25%, were committed by anti-Castro Cubans. The second worst offender, with about 10% of the total, was the Jewish Defense League.
Both these groups flew a bit under the radar, both because they weren't interested in producing mass casualties among the American public and because they have politically powerful protectors.