SPOKANE, Wash. - 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."

Buquet 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."