A federal jury has convicted a man of attempting to produce a biological weapon, a year after he was arrested for possessing explosives and illegal silencers. Police found a large number of castor bean plants, which can be used to make ricin, in Denys Ray Hughes' apartment. Ricin is a poisonous protein that can be used as a biological weapon. Hughes, 59, also was found guilty Friday on charges of possessing a pipe bomb and illegal gun silencers. Authorities said Hughes was working alone and was not a member of a terrorist organization. They also said there was no evidence pointing to specific plans of attack.