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.
UPDATE, 12:00 pm ET: FBI spokesman Peter Donald confirmed Wednesday that agents from his New York office responded to the offices of Viacom after receiving "suspicious mailings."
An unnamed law enforcement official, speaking on condition of anonymity, said the two letters sent to Viacom were addressed to Jon Stewart and Stephen Colbert. The official said letters were also sent to the offices of "several" members of both the House and Senate.
All sources are stressing that no actual danger or threat has been detected and threatening notes are often sent to lawmakers' offices and elsewhere.
-- Fox News' Mike Levine contributed to this report.
After sending out an outdated message in error warning Senate offices of potentially dangerous letters, the Office of the Sargeant at Arms has issued a new email to chiefs of staffs, administrative managers, chief clerks and staff directors warning of suspicious mail.
"Yesterday, a Senate state office received threatening mail that contained a suspicious powdery substance. While this letter was tested and the substance found to be harmless, these incidents are reminders that we need to remain vigilant in handling mail, recognizing suspicious items and knowing what immediate actions to employ if faced with suspicious mail in the office.
"Law enforcement sources have advised us other such letters may be in the mail stream and to be watchful for a particular return address on mail arriving at Senate offices: The MIB, L.L.C., 2413 NW Burnside, Portland, OR 97209."
The reminder is at least the second to come from the Sergeant at Arms office this year. Earlier in the day, the office sent out and then retracted a message to administrative managers and chief clerks that originally dated to January 2011.
The notice of suspicious mail should be disregarded, the email reads, "as the information is no longer current."
In that email, the Sergeant at Arms warned that the Senate Post Office had processed two envelopes containing threatening language and a powdery substance.
"The envelopes were from the same sender and were addressed to two senators’ offices in Washington, D.C.," the warning read.
"The initial tests of the substance were negative," the email continued, reminding offices to remain "vigilant."
The communiqué asked lawmakers' offices to be on the lookout for mail from the Great Meadow Correctional Facility in Comstock, N.Y.
The reminders are ominous echoes from a bioterror attack on two Senate offices shortly after the Sept. 11, 2001, terrorist attacks. Letters were then sent to the offices of two Democratic leaders, Sens. Tom Daschle of South Dakota and Patrick Leahy of Vermont.
Following the 2001 attacks, new procedures for handling mail were put into place to protect staff. All mail bound for congressional offices must first be processed off site. Much of the mail now arrives late and often damaged from the rigorous screening procedures, according to numerous aides.