A Jewish Defense League (search) activist who was killed in a federal prison had received death threats because of his militancy and needed more protection from authorities, his former attorney said Sunday.

Earl Krugel (search), 62, was killed Friday evening at the Federal Correctional Institution (search) in Phoenix, where he was imprisoned for his role in a bomb plot against a Southern California mosque and the office of a Lebanese-American congressman.

Attorney Mark Werksman questioned why Krugel was transferred to the medium-security prison last week. He had been in protective custody at Los Angeles' federal Metropolitan Detention Center in recent years, Werksman said.

"There were death threats against him precisely because he was a militant Jewish activist," he said. "So he always had to be watched and be guarded in federal custody in L.A."

Werksman couldn't recall details of the threats, but he said Krugel might have been targeted by white supremacists or Muslim extremists in the past.

FBI Special Agent Richard Murray said a homicide probe was under way but wouldn't confirm whether officials suspected white supremacists or other inmates. Prison officials referred all calls to the FBI.

Family members said Krugel had been at the Arizona prison for three days and died after an inmate struck him in the head with a concrete block.

Krugel, a former dental assistant from Los Angeles, was 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.

He pleaded guilty to conspiracy charges in 2003, and was sentenced in September to 20 years in prison.

JDL leader Irv Rubin (search), who was also convicted in the case, slashed his neck and fell from a railing at the Los Angeles federal detention center exactly three years to the day before Krugel's death. Rubin died nine days later; his death was ruled a suicide.

The Jewish Defense League was founded in 1968 by Rabbi Meir Kahane. Kahane advocated the forcible removal of Arabs from Israel. He was assassinated in 1990 in New York.