A Jewish extremist who plotted to blow up a mosque in California and the office of a US-Lebanese congressman has been killed in jail.
The plotters targeted a mosque in a Los Angeles suburb
Earl Krugel, 62, died after an assault on Friday at the federal prison in Phoenix, Arizona, the FBI said.
A spokesman said that a homicide investigation had been launched, without giving any further details.
Krugel - a Jewish Defence League member - was jailed for 20 years in September after admitting the plot charges.
Krugel's wife, Lola, said FBI investigators told her an inmate had struck her husband on the head from behind with a cement block, the AP news agency reported.
"Earl never saw it happening. He was exercising," she said.
Krugel was arrested in 2001 with another leader of the Jewish Defence League, Irv Rubin, who died in jail in an apparent suicide.
FBI agents arrested Krugel and Rubin at their homes in November 2001.
Investigators seized bomb-making equipment they believed was destined to be used in an attack on the King Fahd mosque in Culver City, a suburb of Los Angeles.
The pair were charged in December 2001 with conspiring to bomb the mosque and send explosives to the offices of Congressman Darrell Issa, who is of Lebanese Christian descent.
Krugel pleaded guilty to conspiracy to violate the civil rights of worshippers at the mosque and to a weapons charge linked to the plot against the congressman.
In return, he was to escape trial on more serious charges related to the bomb conspiracy.
The plot was discovered before any blasts took place.
The Jewish Defence League was founded in 1968 by a far-right US rabbi, Meir Kahane, who advocated the expulsion of all Palestinians from the West Bank and Gaza Strip.
Kahane was assassinated in New York in 1990.