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Iranian hackers sent messages to people involved in the campaign to re-elect President Joe Biden over the summer, using non-public information from Republican presidential candidate Donald Trump's campaign, US intelligence agencies said on Wednesday.
There is no information indicating those recipients replied, a joint statement from the FBI, the Office of the Director of National Intelligence and the Cybersecurity and Infrastructure Security Agency said.
The statement said Iranian "malicious cyber actors" have continued their efforts since June to send stolen, non-public material associated with the Trump campaign to US media organisations.
The Trump campaign said last month that its communications had been hacked and suggested Iran was to blame. The FBI opened an investigation into the issue and came to the same conclusion.
The hack was first reported by Politico, which said it had begun receiving emails in July containing internal Trump campaign documents from an anonymous account. Mr Trump has said, however, that the information received by the outlet was publicly available. Iran denied involvement.
"Already devoid of any credibility and legitimacy, such allegations are fundamentally unfounded, and wholly inadmissible. The Islamic Republic of Iran does not engage in the internal uproars or electoral controversies of the United States," the Iranian mission to the UN said in a statement, adding that the US should disclose what evidence it may have. "Iran neither has any motive nor intent to interfere in the US election. And it therefore categorically repudiates such accusations."
A Trump campaign spokeswoman said in a statement that the hack was "further proof" Iran is attempting to help Democratic candidate Kamala Harris and Mr Biden, as Tehran knows Mr Trump will "restore his tough sanctions and stand against [its] reign of terror".
US intelligence agencies have warned for months that Iran and other foreign interests are engaging in intense campaigns to influence the outcome of the elections.
The emails sent to the Biden campaign are the latest example of Tehran trying to “stoke discord and undermine confidence in our electoral process”, the statement said.
Election influencing campaigns launched by Iran and others range from fake social media posts to campaign hacks, with varying degrees of success.
“Russia, Iran and China are trying by some measure to exacerbate divisions in US society for their own benefit, and see election periods as moments of vulnerability,” the joint statement said.
“Efforts by these or other foreign actors to undermine our democratic institutions are a direct threat to the US and will not be tolerated.”
