The FBI and the U.S. Postal Inspection Service are investigating suspicious packages that were sent to election officials in six states on Monday, the law enforcement agency said in a statement on Tuesday.

Officials in Nebraska, Iowa, Kansas, Tennessee, Wyoming and Oklahoma were targeted.

The FBI said some of the letters contained an unknown substance that was being examined.

The sender of the letters identified themselves as the "US Traitor Elimination Army," Colorado Secretary of State Jena Griswold said in a post on X.

The threats to election officials come just weeks before Americans go to the polls on Nov. 5 to choose between Democratic presidential candidate Kamala Harris and her Republican rival Donald Trump.

The country is deeply polarized and on edge amid an increase in political violence and two assassination attempts against Trump.

The FBI said it was working to identify those responsible as well as a motive. It was also working to determine how many letters had been sent in total.

Nebraska Secretary of State Robert Evnen's office said its elections division had received a "suspicious" envelope with a substance inside. It said the substance had been tested and found to be non-hazardous.

The threats follow similar letters sent to election officials in several states in November.