UPDATE 2:00pm ET:  Senate Sergeant at Arms Terrance Gainer has just sent an updated e-mail to all Senate offices confirming that three total congressional offices have received threatening mails containing a powdery substance that has initially tested negative for containing a bio-agent and more letters could be on the way.  

But the Gainer warning, obtained by Fox, makes clear, "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. Although all letters received thus far have proved harmless, it is essential that we treat every piece of suspicious mail as if it may, in fact, be harmful." 

A Senate state office and a House district office received the threatening letters Tuesday, and Gainer says an additional Senate state office received a letter Wednesday.  The Sergeant at Arms does not indicate which office received the correspondence, but Gainer repeats the earlier call for members to be on the lookout for a particular Portland, Oregon address previously reported by Fox. 

Gainer says his staff is "working closely with federal and local law enforcement in this ongoing investigation."

Congressional security officials, police, and staff often work with outside law enforcement. Recently, a joint investigation thwarted a would-be suicide bomber who was intent on attacking the U.S. Capitol.  (read more)