A Portland, Ore., man was arrested and charged in a federal court Friday in connection with more than 100 threatening letters sent to congressional offices containing powder that turned out to be harmless.

Christopher Lee Carlson, 39, was arrested at his home in the Portland metro area, the U.S. Capitol Police said Friday night. He was indicted by a federal grand jury on one count of “mailing a threatening communication to a member of Congress” and another count of “mailing a letter threatening to use a biological weapon to a U.S. senator.”

Carlson is in custody and expected to be arraigned on Monday, the FBI said.

“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.

On Feb. 22, Senate Sergeant-At-Arms Terrance Gainer warned congressional staffers to be on alert after two Senate state offices and a House district office received what Gainer called “threatening” letters.

The Associated Press last month reported that a district office of House Speaker John Boehner (R-Ohio) was among the recipients. The letters reportedly stated that there is a “10 percent chance you have just been exposed to a lethal pathogen.”

Though the letters were deemed harmless, “it is essential that we treat every piece of suspicious mail as if it may, in fact, be harmful,” Gainer said in the warning to congressional offices.

The Capitol Police worked with the FBI and U.S. Postal Service on the investigation that led to Carlson’s arrest Friday morning. No other information was given about the suspect.

(Source: Politico)