Registered Democrats in Florida and Alaska have received emails in the past week threatening them with reprisals if they vote "wrong."
"You vote for Trump on election day, otherwise we will," it says in the email that was allegedly sent by the pro-violence The Proud Boys, which was singled out as a hate group targeting blacks, Muslims, Jews and women.
The security service now says that the email instead came from Iran, and is part of the influence the country is trying to have on the election on 3 November.
"We have confirmed that Iran and Russia, respectively, have come across some data on the registered voters," John Ratcliffe said at a news conference.
Ratcliffe: "Scare Tactics"
- These are desperate actions taken by desperate opponents.
Some of the current emails also contain a video where it is alleged that false votes were cast in the election and how you can submit false ballot papers yourself.
"That video, and all the allegations about false ballots, are untrue," Ratcliffe said, adding that Iran was acting to intimidate and harm President Donald Trump with intimidation tactics.
Has been accused before
Russia has previously been accused of trying to influence the outcome of the election through a propaganda campaign on social media and in the media ahead of the 2016 presidential election.
Among other things, the country is said to have wanted to undermine voters' confidence in the US electoral system.
FBI Director Christopher Wray assures at the same press conference that the US election system is safe.
- We will not allow foreign interference in our elections and all criminal activities that threaten the election security or confidence in it, he says.
- You can trust that your votes count.