According to the statement this week, the FBI and U.S. Postal Inspection Service are now looking into a group of “strange” letters sent to election officials across the country. Some of the letters contained an unknown substance.

Several secretaries of state and state voting offices in Kansas, Nebraska, Oklahoma, Tennessee, Wyoming, and Iowa received the strange letters, according to the Associated Press, which cited officials from those states.

In comments to CBS News, the FBI and U.S. Postal Inspection Service said they were looking into a number of strange mails sent to poll officials in several states. “Some of the letters had an unknown material inside, and we are working closely with law enforcement to handle each case and get the letters to the right place safely.”

The agencies said, “We are also working with our partners to find out how many letters were sent, who sent them, and why they were sent. Because the review is still going on, we won’t say anything else about it, but the public can be sure that safety is our top concern. Also, we want to warn everyone to be careful when opening mail, especially from people you don’t know. Please call the police right away if you see something that doesn’t seem right.”

The letters are a worry because the federal government and cyber security experts have already told state election officials that they could face threats during the 2024 election season.

NBC News got a copy of a report from the FBI and the U.S. Postal Inspection Service on Monday that said, “Law enforcement is working hard to intercept any new letters before they travel.”

The letter said, “People who work with elections should stay alert and be careful when handling mail. All substances that seem odd should be thought of as possibly dangerous and should be handled according to set safety rules.”

CBS News reports that Michigan Secretary of State Jocelyn Benson said she is “getting threats, whether it’s through voicemails, emails, social media, or in person.”