Iranian intelligence was responsible for a recent campaign of emails sent to intimidate Florida voters, the FBI announced Wednesday night, adding that Russia was also working to influence the election.
Iranian and Russian officials denied the charges.
The emails, which ominously instructed Democratic voters in Florida to switch to the Republican Party, purported to come from the "Proud Boys", a right-wing group of Trump supporters that became a flashpoint during the first presidential debate.
But the emails were actually "spoofed" and had been designed "to incite social unrest and damage President Trump," said John Ratcliffe, the director of national intelligence. Ratcliffe did not explain how the emails were damaging to Trump, because they were urging Democrats to switch to the Republican Party.
Ratcliffe didn't provide any evidence for the attribution.
Many states, including Florida, make voters' information, including their names and party affiliations, easily accessible to members of the public who request it.
Both Iran and Russia had obtained some Americans' voter registration information, Ratcliffe said.
In the same news conference, FBI Director Christopher Wray said there was still no way for Iranian or Russian intelligence to change Americans' votes.
"You should be confident that your vote counts," Wray said.
Late Wednesday Alireza Miryousefi, a spokesman for the Iranian Mission to the U.N., also denied that the country had taken any action to influence the U.S. election.
"Unlike the U.S., Iran does not interfere in other country's elections," he said in a statement.
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"The world has been witnessing U.S.'s own desperate public attempts to question the outcome of its own elections at the highest level. These accusations are nothing more than another scenario to undermine voter confidence in the security of the U.S. election and are absurd."
On Thursday, Iran summoned the Swiss envoy in Tehran to protest against what it called "baseless" U.S. claims that Tehran has tried to interfere with the presidential election in November.
Switzerland represents U.S. interests in Iran because Washington and Tehran have no diplomatic ties.
"Iran's strong rejection of American officials' repetitive, baseless and false claims was conveyed to the Swiss ambassador," Foreign Ministry spokesman Saeed Khatibzadeh told Iranian state TV.
Meanwhile, Kremlin spokesman Dmitry Peskov told reporters in a press briefing on Thursday: "The accusations are poured out every day, they are all absolutely groundless, they are not based on anything, rather it is a tribute to the internal political processes associated with the upcoming election."
Iran has increased its online influence operations in recent years, often following Russia's playbook to create partisan news sites that support the Iranian government's worldview — like opposition to Israel and Trump — with little traction.
But this operation, a more active trick to falsely accuse some of Trump's most infamous supporters, represents a new front for Iranian operations, said John Hultquist, who heads intelligence analysis at the cybersecurity company Mandiant.
"This incident marks a fundamental shift in our understanding of Iran's willingness to interfere in the democratic process," Hultquist said in an email. "While many of their operations have been focused on promoting propaganda in pursuit of Iran's interests, this incident is clearly aimed at undermining voter confidence."
After the announcement, a spokesperson for Google said in a statement the company had seen "evidence that an operation linked to Iran sent inauthentic emails to people in the U.S. over the past 24 hours."
It added that automated spam filters for Gmail users had stopped 90 percent of the approximately 25,000 emails sent on its platform.
"We referred the matter to the FBI and will continue to work with law enforcement and others in the industry to identify and remove any related content," the spokesperson said Thursday.
U.S. election officials have repeatedly said the measures the government has taken ahead of the 2020 election in response to Russia's interference in the 2016 election will make this the most secure in modern history.