The FBI and U.S. Postal Inspection Service are investigating a series of suspicious letters, some of which contained an unknown substance, sent to election officials nationwide, the agencies confirmed Tuesday.

The Associated Press reported powder-containing packages were sent to secretaries of state and state election offices in Iowa, Kansas, Nebraska, Oklahoma, Tennessee and Wyoming, citing officials in those states.

"The FBI and U.S. Postal Inspection Service are investigating a series of suspicious mailings sent to election officials in several states," the agencies said in statements to CBS News. "Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident and safely collect the letters."

"We are also working with our partners to determine how many letters were sent, the individual or individuals responsible for the letters, and the motive behind the letters," the agencies said. "As this is an ongoing matter we will not be commenting further on the investigation, but the public can be assured safety is our top priority. We would also like to remind everyone to exercise care in handling mail, especially from unrecognized senders. If you see something suspicious, please contact law enforcement immediately."

Officials in at least three of the states — Oklahoma, Tennessee and Iowa — said they determined that there was no threat posed by the substances. A spokesperson for the Tennessee secretary of state's office said the substance "tested negative for hazardous materials." The Oklahoma Board of Elections said the substance was found to be flour.

The suspicious letters come after federal law enforcement and cybersecurity officials warned state election administrators that they face serious threats this election season. Election officials from battleground states told CBS News recently that threats are escalating ahead of November.

Michigan Secretary of State Jocelyn Benson, a Democrat, told CBS News, "We're daily receiving threats, whether it's through voicemails, emails, social media or in person."

Benson said she personally is receiving threats, "and it's escalating."

"They're all rooted in lies and misinformation, which is always disappointing and sad, but at the same time, it's real," she said.

Alexandria Johnson contributed reporting.