A 37-year-old janitor who has been charged with threatening to kill a federal judge in a case that involves letters containing the deadly poison ricin is a registered sex offender who lived in a rundown apartment building near downtown Spokane.

Matthew Ryan Buquet remained jailed Thursday after appearing in federal court a day earlier and pleading not guilty to a charge of mailing a threatening communication.

"He sticks to himself," said Scott Ward, who lives across the hall from Buquet's apartment. "He doesn't talk, really. He's kind of quiet."

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.

Little information was immediately available about Buquet. His public defender did not immediately return a telephone call Thursday.

Buquet is a registered sex offender from a 1998 conviction for taking indecent liberties with a 10-year-old girl, according to the Spokane County Sheriff's Office. He served 18 months in jail.

A search of public records shows Buquet doesn't appear to have any close relatives.

A Facebook page with his name and photo says he's originally from Bogota, Colombia, and studied electronic engineering technology at ITT Technical Institute in Spokane Valley. It says he worked for ABM Janitorial Services.

A message left at ABM's Spokane office Thursday was not immediately returned, but the company issued a statement through a public relations firm: "The individual in question has not been employed by our company for more than a month, long before these letters were reportedly postmarked. We are cooperating with the authorities and, given that this is a pending case, at this point we would refer you to them."

Wednesday's grand jury indictment accuses Buquet 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."

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.

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.

He will next be in court for a bail hearing Tuesday. If convicted of mailing a threatening communication, he could face up to 10 years in prison and a $250,000 fine.

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. The recipe for making the poison is readily available on the Internet.

There were no reports of illness connected to the Spokane letters.

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.

The FBI has no information indicating the two ricin cases are related, said agency spokeswoman Ayn Sandalo Dietrich.