Congressional offices were targeted by mail containing suspicious powder, the Senate Sergeant at Arms was quoted as saying Wednesday.

In an email to Congressional offices, Senate Sergeant at Arms Terry Gainer warned staffers to be on the lookout for letters containing "a suspicious powdery substance," according to the Hill newspaper, which specializes in reporting the Congress.

The email said a Senate State office and a House District office received on Tuesday threatening mail that contained the substance. The names of the lawmakers whose offices were targeted have not yet been disclosed.

According to Gainer, the letters received were tested and the enclosed substance found to be harmless. But there were threats of more letters to come.

Gainer said his office was working closely with both federal and local law enforcement during the ongoing investigation.

Last week, a terror suspect has been arrested near the Capitol as part of an anti-terrorism investigation. The suspect, 29-year old Amine El Khalifi, was nabbed by the FBI in a one-year sting. He was carrying what he believed to be explosives to carry out an attack on the capital.