(AP) -- A 39-year-old man charged in connection with threatening letters containing white powder that were sent to members of Congress and some media organizations was arrested Friday.
Investigators said Christopher Lee Carlson was indicted on two criminal counts arising out of an investigation into the mailing of about 100 envelopes containing white power.
The letters, postmarked in Portland, Ore., so far have all tested negative for toxic substances, the U.S. Attorney's office in Portland said.
A federal grand jury indictment returned Friday in Portland charged Carlson, described as being from the Portland-Vancouver, Wash., area.
The U.S. Capitol Police said in a statement that Carlson was arrested at a home in the Portland area.
Carlson was charged with one count of mailing a threatening communication to a member of Congress. House Speaker John Boehner, R-Ohio, was referenced by name. The second count charged the man with mailing a letter threatening to use a biological weapon to a U.S. senator. Sen. Barbara Mikulski, D-Md. was named in that count.
Carlson is expected to be arraigned Monday.
Investigators have recovered more than 100 letters addressed to U.S. senators and representatives. The Seattle office of Sen. Patty Murray, D-Wash., said it also received one.
"Threatening letters - whether hoax or real - are serious concerns that federal law enforcement agencies will aggressively pursue," said Greg Fowler, special agent in charge of the FBI in Oregon.
Murray's office did not immediately return a call for comment Friday night.
Some letters were sent to district offices of the Congress members.
The FBI, U.S. Capitol Police and the U.S. Postal Inspection Service all participated in the investigation.
The letters, which first came to light in late February, told the recipients that there is a "10 percent chance you have just been exposed to a lethal pathogen."
The letters bore a return address from "The MIB." The listed Portland return address didn't exist.
The sender wanted an "end to corporate money and `lobbying,'" an end to corporate "personhood" and a new constitutional convention. The Associated Press obtained a copy of a letter.
The threats raised memories of post-9/11 incidents that rattled Washington. In mid-November 2001, authorities closed two Senate office buildings after anthrax attacks on Congress. Those attacks came after four people - two postal workers in Washington, a New York City hospital worker and a Florida photo editor - died from exposure to anthrax.