(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 weren'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.