A Jewish Defense League activist imprisoned for his role in a plot to bomb a California mosque and the office of a Lebanese-American congressman was killed at a federal prison in Phoenix, an FBI spokesman said Saturday. Earl Krugel, 62, was killed in an assault Friday evening at the Federal Correctional Institution, said FBI agent Richard Murray. Murray wouldn't release further details but said federal authorities had opened a homicide investigation. Earl Krugel, a former dental assistant from Los Angeles, and late JDL leader Irv Rubin were arrested in 2001 and charged with conspiring to bomb the King Fahd Mosque in Culver City and a field office of Republican Rep. Darrell E. Issa, who is Lebanese-American. Rubin died in 2002 after plunging 18 feet over a railing at a downtown jail in what authorities ruled a suicide. Krugel pleaded guilty the following year to one count of conspiracy to violate the civil rights of worshippers at the mosque and one count of carrying an explosive device in connection with a conspiracy to impede or injure an office of the United States.