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 . NEW : Matthew Buquet will stay in custody until at least next Tuesday , FBI says . Comment: Tests showed ricin was found with the threatening letter , the FBI says . He was arrested Wednesday afternoon in Washington state .
NEW : Matthew Buquet will stay in custody until at least next Tuesday , FBI says . Comment: Tests showed ricin was found with the threatening letter , the FBI says . He was arrested Wednesday afternoon in Washington state .