Washington, Feb 23 (IANS): Two officials of the US Congress have received letters that contained a "suspicious powdery substance", following which officials sent out a warning cautioning other staffers to be on the lookout.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", Xinhua reported citing the Hill newspaper, which specialises in reporting the Congress.

The email said a Senate State office and a House District office received threatening mail Tuesday that contained the substance.

The names of the lawmakers whose offices were targeted were not disclosed.

Gainer said the letters were tested and the enclosed substance found to be harmless.

But there were threats of more letters to come, the report said.

Gainer said his office was working closely with both federal and local law enforcement officials in an investigation into the incident.

Recently, a Moroccan terror suspect was arrested for plotting an attack near the Capitol.