Three US lawmakers received threatening letters containing a harmless white powder, but the sender said more missives including a 'harmful material' could follow, a Senate official said on Wednesday.
The anonymous sender "has indicated that additional letters containing a powdery substance will be arriving at more Senate offices," Senate sergeant-at-arms Terrance Gainer said in a email to staff.
"Some of these letters may contain an actual harmful material," he added, noting the missives were postmarked from Oregon.
Similar letters, which included complaints about corporate influence over US politics, also were received by several US media outlets, but they did not contain any white powder, said law enforcement officials.
The letters were received by organisations such as The Washington Post, The New York Times, USA Today, Fox News and National Public Radio.
CBS News quoted a law enforcement official as saying that in missives to comedians Jon Stewart and Stephen Colbert, the sender warned that 100 letters were sent to the Washington or home-state offices of US senators.
Of those 100 letters, 10 of them would contain a deadly pathogen, according to the official.
The FBI said in a statement that it was "aware of this situation and is responding accordingly."
"So far, none of the letters have contained a hazardous substance. We are working with those law enforcement agencies affected to determine if the mailings are related," the FBI added.
"We take these matters seriously and will investigate fully."
The letters raised alarms because of the 2001 anthrax attacks in which anonymous letters containing the deadly pathogen were sent to offices of two Democratic senators and several media offices, one week after the September 11 attacks. Five people were killed and 17 others infected by the anthrax spores.