US authorities on Wednesday said Iranian cyberattackers had offered "stolen, non-public" material from Republican presidential candidate Donald Trump's campaign to staff for his then White House rival, Joe Biden.
US intelligence and law enforcement agencies said the hackers "sent unsolicited emails to individuals then associated with President Biden's campaign that contained an excerpt taken from stolen, non-public material from former president Trump's campaign."
Biden was at the time the Democratic presidential nominee, before dramatically stepping aside in July and endorsing his vice president, Kamala Harris.
The joint statement from the US Office of the Director of National Intelligence, the Federal Bureau of Investigation, and the Cybersecurity and Infrastructure Security Agency said none of Biden's campaign staff replied to the emails.
In August, the same agencies first attributed the hack to Iran, accusing Tehran of seeking to influence the 2024 election. Iran denies the allegations.
"Foreign actors are increasing their election influence activities" as Election Day in November approaches, the US statement said, singling out Russia, Iran and China as "trying by some measure to exacerbate divisions in US society for their own benefit."
The US agencies said the Iranian cyberattackers had also attempted to share the information stolen from the Trump campaign with US media organizations. It did not name the outlets.
On Wednesday, the Iranian mission to the United Nations vehemently denied the allegations.
"Already devoid of any credibility and legitimacy, such allegations are fundamentally unfounded, and wholly inadmissible," it said in a statement.
"Iran neither has any motive nor intent to interfere in the U.S. election; and, it therefore categorically repudiates such accusations."
The United States goes to the polls on November 5, with both the Trump and Harris campaigns saying they have been targeted by cyberattacks in recent weeks.
US-based tech companies have also said they detected such attacks.
Trump's campaign on Wednesday said the plot attributed to Tehran was "proof the Iranians are actively interfering in the election to help Kamala Harris and Joe Biden."
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Trump campaign press secretary Karoline Leavitt said this was "because they know President Trump will restore his tough sanctions and stand against their reign of terror."
In 2016, a hack of Democratic National Committee emails -- blamed on Russians -- exposed internal party communications, including about candidate Hillary Clinton.
Trump, who would go on to win that election, was criticized for encouraging the hack.
Harris's campaign said on August 13 that it too had been targeted by foreign hackers, but did not give an indication of which country was believed to be behind the attempt.