( CNN ) -- A 37-year-old man arrested Wednesday in Washington state as part of an investigation of ricin-laced letters threatened in one such letter to injure and kill a federal judge , a grand jury indictment alleges . FBI agents arrested Matthew Ryan Buquet on Wednesday afternoon , and he made his initial court appearance in Spokane later in the day , the federal agency 's Washington state office said in a news release . Buquet remained in custody after the appearance , and he 'll stay behind bars at least until a bail hearing takes place next Tuesday afternoon , FBI spokeswoman Ayn Dietrich said . A grand jury charged Buquet with mailing threatening communication , claiming he `` knowingly and willfully '' mailed through the U.S . Postal Service a letter `` containing a threat to injure and kill Judge ( Fred ) Van Sickle , '' according to the indictment . Van Sickle is a senior judge in the U.S. District Court for the Eastern District of Washington . In a subsequent news release , the FBI said tests -- conducted by that agency and the Spokane Regional Health District -- showed that a suspicious substance found with the letter was `` active ricin toxin . '' `` Our coordinated team acted swiftly to resolve a potentially dangerous situation , and continues working tirelessly around the clock to investigate the origin of the letters and to address any remaining , potential risks , '' said Laura Laughlin , the FBI 's special-agent-in-charge at the bureau 's Seattle office . The U.S . Postal Inspection Service has been trying to find the source of the two letters containing ricin since they were intercepted in a screening procedure at a postal facility in Spokane , the FBI said last week . The American Postal Workers Union earlier did say a ricin-laced letter was sent to a judge in Spokane , the northeastern Washington city in which Van Sickle works . Another letter reportedly containing the toxin was directed to the Spokane Post Office , the union said . Both letters were postmarked May 14 . Postal Service management told employees they were n't at risk from handling the letters because the suspected substance was not in a form that could be inhaled or readily ingested , the union said . That said , preliminary lab tests did indicate it was indeed ricin . If inhaled , injected or ingested , less than a pinpoint of ricin can kill a person within 36 to 48 hours because of the failure of the respiratory and circulatory systems . There is no known antidote for the toxin , which is derived from castor beans . CNN 's Amanda Watts contributed to this report .
NEW : John Smith will remain in custody until at least next Tuesday , it says . The 37-year - old threatened to ` ` " and kill '' a federal judge , an indictment alleges . Tests showed DNA was found with the threatening letter , the FBI says . He was arrested yesterday afternoon in Washington DC .