"While none of the mail received and tested thus far has been found to be harmful, it is clear that the person sending these letters is organized and committed, and the potential to do harm remains very real," Senate Sergeant-at-Arms Terrance Gainer said in a Thursday letter to the Senate community.
Gainer has also said more letters could be on the way.
In statements to Roll Call, spokesmen for Coats and Roberts said the Senators were out of the office during the scares and that testing at Coats' office showed the substances were harmless.
Lieberman spokeswoman Whitney Phillips described the Hartford office's mailing as a "package" that prompted a decision to ultimately evacuate the office and have district staff telecommute on Friday.
Initial tests of the powdery substance came back negative for hazards, Phillips continued; further testing will illuminate what the substance actually is.
Back in Washington, the Capitol Rotunda briefly shut down Friday morning when a mysterious powder was detected on the floor. Capitol Police put the substance through routine testing and it was found to be just smashed-up candy from a candy necklace, Schneider said.
Former Sen. Scott Brown, R-Mass., candidate for U.S. Senate in New Hampshire, holds his hand over his heart during the singing of the national anthem as he waits to take the stage for his town hall campaign rally with Sen. John McCain at the Pinkerton Academy in Derry, N.H., on Monday, Aug. 18, 2014.