State and federal authorities have three men in custody here in what investigators describe as a possible terrorist plot by an obscure Islamic group in California state prisons. FBI officials, who are overseeing the investigation, say they are not convinced that the three men posed a serious threat of violence on their own. But the bureau is continuing to investigate the possibility that they were part of a larger group planning to attack National Guard installations, synagogues and other locations in Southern California. The latest suspect to be arrested is Hamad Riaz Samana, 21, a Pakistani citizen who was taken...