WASHINGTON — Several US lawmakers received threatening letters containing a harmless white powder, but the sender warned more missives including a “harmful material” could follow, a Senate official said.

The news sparked alarm and served as a grim reminder of the 2001 anthrax attacks in which letters containing the deadly pathogen were sent to offices of two Democratic senators and several media offices. Five people were killed.

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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 Portland, 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 quoted by US media.

The letters were received by organizations such as The Washington Post, The Los Angeles Times, USA Today, Fox News and National Public Radio, reports said.

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.

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Of those 100 letters, 10 of them would contain a deadly pathogen, according to the official.

Although law enforcement officials did not identify any of the lawmakers who received the letters, media reports indicate one of them was received by the office of Senator Patty Murray, a Democrat from the state of Washington.

The FBI said in a statement that it was “aware of this situation and is responding accordingly.”

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“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.”

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The letters were signed “The MIB, LLC” and gave a Portland street address. Although the FBI confirmed some of the letters were postmarked from Portland, the street address listed on them was false.

The 2001 anthrax attacks, which came just a week after the September 11 attacks on New York and Washington, forced the evacuation of two congressional office buildings for weeks so they could be decontaminated.

All mail to Congress was quarantined until it was tested for anthrax, and new equipment was installed to scan the mail for contaminants.