Threatening letters — sent with a “suspicious powdery substance” — have been sent to the offices of three members of the U.S. Congress.

Law enforcement authorities said Wednesday they had tested the substance and found it harmless. But in an e-mail to congressional staffers, Senate Sergeant-at-Arms Terrance Gainer warned more letters may be on the way.

Gainer, the Senate's top law enforcement agent, said the person sending the letters had indicated that some of them may contain actual harmful material.

Officials have not released the names of the recipients, but say two letters went to Senate offices and one to

the home state office of a House representative. Gainer warned that special attention should be paid to letters postmarked from Portland, Oregon.

An unnamed law enforcement official also says suspicious letters were sent to comedians Jon Stewart and Stephen Colbert. The letters were mailed to the New York offices of Viacom, which produces the television news satire shows that Stewart and Colbert anchor.

The letters come just a week after federal authorities arrested a man accused of plotting to bomb the U.S. Capitol.