Federal investigators are racing to trace a burst of suspicious letters mailed to Capitol Hill offices and state election headquarters across the country, after several envelopes containing an unidentified white powder triggered evacuations, hazmat callouts and FBI testing this week.

The U.S. Postal Inspection Service and the FBI said letters turned up at six Democratic Senate offices, including that of Minority Leader Chuck Schumer, as well as Sens. Kirsten Gillibrand, Chris Murphy, Alex Padilla, Chris Coons and Lisa Blunt Rochester. Election offices in Delaware, Louisiana, Oklahoma and Wisconsin reported opening envelopes with powder inside, while similar mail bound for Pennsylvania, Maryland, New Hampshire, Ohio and Mississippi was intercepted before it reached election workers, according to CBS News.

"Any threat or attack on an election official is unacceptable," USPIS deputy chief inspector Brendan Donahue told reporters, adding that the letters were sent to "various addresses nationwide" and that investigators have not yet determined who is responsible or why.

No injuries, but offices on edge

So far, nobody has been hurt. Louisiana Secretary of State Nancy Landry said tests "found no evidence of harmful substances," and the Wisconsin Elections Commission reported that "field testing confirmed no hazardous material was detected." Oklahoma's election board evacuated its building as a precaution before giving the all-clear, while a letter addressed to Murphy's Connecticut office was intercepted by postal screeners before it ever reached his staff, according to his spokesman.

"THIS INCIDENT WILL NOT SHAKE US" — Wisconsin elections administrator Meagan Wolfe, per Votebeat

Arizona's office was also evacuated after a powder-filled letter arrived, and Secretary of State Adrian Fontes said his staff wouldn't be "intimidated out of doing its job for voters." Ohio Secretary of State Frank LaRose called the mailings "cowardly." Officials briefed on the investigation told Votebeat that more than 100 letters may have gone out nationwide to a mix of politicians, election offices and at least one polling organization, though the FBI has declined to confirm a total count.

The timing has rattled election officials already bracing for a contentious midterm cycle. NBC News reported the mailings surfaced less than three weeks before Election Day, echoing a similar spree of powder-laced letters sent to election offices in six states ahead of the 2024 vote — incidents that also turned out to involve non-hazardous substances.

The House sergeant at arms said no House members have been targeted so far. Capitol Police and the Senate sergeant at arms have issued fresh mail-screening guidance to congressional staff, while the FBI said it is "working with law enforcement partners" state by state to process each envelope through its lab. No arrests have been announced.