The U.S. attorney's office for Eastern Washington had no comment on whether additional charges might be sought.
Matthew Ryan Buquet, 37, was arrested Wednesday.
A grand jury indictment accused him of mailing a death threat to U.S. District Judge Fred Van Sickle in Spokane on May 14.
The indictment did not mention ricin, but the FBI made the link in a news release late Wednesday, saying analysis showed the letter sent to the judge contained "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," Laura Laughlin, the special agent in charge of the FBI's Seattle office, said in a statement.
The U.S. Postal Service said last week that two letters were intercepted -- one addressed to the courthouse and the other to the downtown post office -- and they contained ricin in a crude form that did not immediately pose a threat to workers.
Buquet appeared in federal court in Spokane after the FBI said agents arrested him Wednesday afternoon. He pleaded not guilty to mailing a threatening communication.
The short, balding Buquet wore dark-tinted glasses and was shackled in court. He gave brief "yes" and "no" answers to questions from U.S. Magistrate Judge Cynthia Imbrogno.
A search of federal court records turned up no indication that Buquet had ever appeared before Van Sickle or had any connection to the judge.
Imbrogno ordered Buquet held until a bail hearing scheduled for Tuesday. A public defender was appointed to represent him.
If convicted of mailing a threatening communication, he could face up to 10 years in prison and a $250,000 fine.
U.S. Attorney Michael Ormsby declined to comment after the hearing, and little information about Buquet was immediately available.
Ricin is a highly toxic substance made from castor beans. As little as 500 micrograms, the size of the head of a pin, can kill an adult if inhaled or ingested.
There were no reports of illness connected to the Spokane letters.
Investigators in hazardous materials suits spent most of Saturday executing a search warrant at a three-story apartment building in downtown Spokane. Witnesses reported that agents escorted a man from the building.
The Spokane investigation comes a month after letters containing ricin were addressed to President Barack Obama, a U.S. senator and a Mississippi judge. A Mississippi man was arrested in that case.
Associated Press writer Gene Johnson in Seattle contributed to this report.
Copyright 2013 The Associated Press.