Letters containing a powdery substance were sent to at least three congressional offices this week, and the sender is threatening more that could include harmful material, the top Senate law enforcement official said Wednesday.

In a message to the U.S. Senate community, Sergeant at Arms Terrance Gainer said three letters that contained harmless powder showed up at two Senate state offices and a House district office on Tuesday and Wednesday.

"The author of these letters has indicated that additional letters containing a powdery substance will be arriving at more Senate offices and that some of these letters may contain an actual harmful material," Gainer's message said.

While the three letters received so far were harmless, "it is essential that we treat every piece of suspicious mail as if it may, in fact, be harmful," Gainer continued in his message.

Any letters postmarked from Portland, Oregon, required special attention, especially those with a particular Portland return address, Gainer's message said. The Portland return address cited by Gainer did not appear to exist.

"If any mail is received from this return address, it should remain unopened and the local authorities contacted immediately, followed by notification to the United States Capitol Police Threat Assessment Section," Gainer's message said.

Federal and local law enforcement authorities are investigating the possible threat, according to Gainer.

While phone and letter threats to congressional offices are relatively commonplace, precautions against a possible toxic chemical attack increased after letters containing anthrax sent to congressional offices and news organizations killed five people and sickened 17 others in 2001.