The FBI is investigating what it calls a “series of suspicious mailings” after election officials in multiple states reported receiving unusual things in the mail.

The U.S. Postal Inspection Service told Scripps News in a statement, “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.”

Arizona Secretary of State Adrian Fontes told Scripps News his state has security measures in place as at least 15 states have reported receiving suspicious packages.

“It’s not just affecting election officers,” Fontes said. “It’s affecting the post office.”

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“Safety is a really big deal for us, in fact I was the first election officer in the nation to have an election security officer working with us,” he added. “That’s when I was the Maricopa County recorder.”

Voters could lose right for lack of documentation provided

In Arizona’s Maricopa County, around 100,000 voters may not be able to vote in state and local races after not providing certain documentation to prove citizenship. Fontes calls the rules in Arizona “extreme”

“In Arizona we have a bifurcated system,” Fontes told Scripps News. “If you show documented proof of citizenship you can vote a full ballot. If you sign off on the affidavit, under penalty of perjury like every other American does, you’re only allowed to vote in federal races. President, Senate and Congress.”

He added that the state is asking the Arizona Supreme Court if those involved might have to be moved over to federal only ballots or if they can vote in every election for which they would usually be eligible.

You can watch Scripps News’ full interview with Arizona Secretary of State Adrian Fontes in the video player above.