By Teresa Carson
PORTLAND, Oregon (Reuters) - A registered nurse from the Portland area pleaded not guilty on Thursday to additional charges stemming from more than 100 threatening letters he is suspected of sending to members of Congress and the media in February.
Christopher Carlson, 39, has been jailed without bond since his arrest at his home in Vancouver, Washington, on March 9 in the mass mailing of envelopes postmarked from Portland, Oregon, that contained a white powder later determined to be harmless.
The envelopes triggered a security alert on Capitol Hill and among several media outlets when they turned up over a two-day period in February.
The 10 new felony counts against Carlson were contained in an expanded indictment returned by a federal grand jury on March 28, just after he entered not guilty pleas to two earlier charges.
Carlson has now pleaded not guilty to 12 charges - six counts of mailing a threatening communication to a member of Congress and six counts of mailing a threat to use a biological weapon.
The threatening letters were addressed to House of Representatives Speaker John Boehner, an Ohio Republican; Senator Barbara Mikulski, a Maryland Democrat; and four Republican senators - Marco Rubio of Florida, Pat Roberts of Kansas, David Vitter of Louisiana and Ron Johnson of Wisconsin.
If convicted, Carlson would face a maximum penalty of 90 years in prison.
More than 100 threatening letters were received by various lawmakers and media offices, including The New York Times and National Public Radio, starting on February 22. It was not clear whether prosecutors would seek additional charges.
Authorities did not offer a possible motive.
Carlson worked as a registered nurse and then as a trainer in electronic recordkeeping at a Vancouver hospital until he took a leave of absence in December. He sat calmly at the defense table during the five-minute hearing on Thursday.
A trial date has been set for June 5.
February's security alert marked the biggest postal scare in the nation's capital since deadly anthrax-laced letters were sent to several news organizations and Senate offices in 2001 in the wake of the September 11 attacks on America.
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.
(Editing by Steve Gorman and Doina Chiacu)