Several members of the U.S. Congress have received threatening mail containing suspicious powder, local media reported, quoting the Congress chief law enforcement officer.

The powder was found to be harmless, but Senate Sergeant at Arms Terry Gainer warned congressmen to be on the lookout for letters containing “a suspicious powdery substance,” adding that special attention should be paid to letters sent from Portland, Oregon, reports said.

A number of U.S. news organizations have received letters warning that similar mail has been sent to the Washington or local offices of all 100 U.S. senators and that 10 of them contained a deadly pathogen.

At least three senators said they had received letters with a powdery substance, which proved to be not dangerous, media reports said.

Police, the Federal Bureau of Investigation (FBI) and other special services have launched an investigation into the incidents, Gainer said.

Reuters quoted a law enforcement source as saying the letters sent to the news organizations contained no suspicious substances. Signed “the MIB,” they demanded an end to corporate money and lobbying in U.S. politics, an end to corporate personhood, and called for a new constitutional convention.