A man was arrested Friday for sending threatening letters containing a white powder to members of Congress, U.S. Capitol Police said.

The man, Christopher Lee Carlson, 39, was arrested at his home in the Portland-Vancouver metro area Friday morning, Capitol Police said.

Carlson allegedly sent a slew of letters to lawmakers in the House and Senate that contained the white powder. Tests have concluded that the powder is not hazardous.

The man was charged with sending threatening communications to a member of Congress and threatening to use a biological weapon, according to Capitol Police.

Capitol Police worked with the FBI and the U.S. Postal Inspection Service in recent weeks to arrest the man.

Senate Sergeant at Arms Terry Gainer warned lawmakers’ offices in February about the letters, which he said had also been sent to several district offices.