Nurse pleads not guilty to congressional mail threat PORTLAND, Oregon (Reuters) - A nurse from the Pacific Northwest pleaded not guilty on Wednesday to charges stemming from more than 100 threatening letters he is suspected of sending to members of Congress and the media last month. Christopher Carlson has been jailed without bond since his arrest at his home in Vancouver, Washington, on March 9 in connection with the mass mailing of menacing envelopes postmarked from nearby Portland, Oregon, that contained a white powder later determined to be harmless. The envelopes, which turned up over a two-day period in February, triggered a security alert on Capitol Hill and among several media outlets. Carlson, 39, entered his not-guilty plea in federal court in Portland to one count of mailing a threatening communication to House Speaker John Boehner, an Ohio Republican, and one count of mailing a threat to use a biological weapon to U.S. Senator Barbara Mikulski, a Maryland Democrat. If convicted of both offenses, Carlson faces a combined maximum sentence of 15 years in prison. Prosecutors said additional charges will likely be brought against him in a new indictment they expect will be returned by his next court hearing, set for April 5. Wearing a green thickly padded suicide smock and shackled at the wrists and ankles, Carlson spoke quietly with his attorney during the proceedings, raising his voice loud enough to be heard only to say he understood Judge Dennis Hubel's explanation of his rights. Neither the original two-count indictment nor prosecutors have made any mention of a possible motive for the threatening correspondence. Carlson had worked at the Legacy Salmon Creek Medical Center in Vancouver as a registered nurse from November 2008 until July 2010, when he took a job at the hospital training other staff how to use a new electronic record-keeping system, hospital spokesman Brian Terrett said. Terrett said Carlson took a leave of absence from the hospital in December 2011 and was considered an employee in good standing at that time. He declined to elaborate further on Carlson's employment status due to personnel privacy policies. After the hearing, U.S. Attorney Stephen Peifer declined to elaborate on the potential for additional charges. The letters with powder in the envelopes began to show up at lawmakers' offices in Washington, D.C., and their home states on February 22. The incident rattled nerves in the nation's capital, where memories remain fresh of deadly anthrax-laced letters that were sent to several news organizations and Senate offices in 2001, in the wake of the September 11 attacks on the World Trade Center and Pentagon. Five people were killed and 17 sickened by those letters, which federal investigators said were ultimately traced to a U.S. Army scientist who committed suicide in 2008.