Iranian hackers distributed hacked information about Donald Trump's electoral campaign to people linked to the Biden campaign, according to the FBI and US intelligence agencies.
US officials now believe that information taken from the Trump campaign was sent in unsolicited emails to people linked to the campaign in late June and early July - before Biden dropped out of the race for president.
There is currently no evidence that the hackers received any reply from any recipients.
In August, officials warned that Iran hopes to "stoke discord" and undermine confidence in US institutions ahead of the November election.
US officials said that Iran had used “social engineering and other efforts” to seek out direct access to both the Democratic and Republican campaigns – a tactic they said had been used by both Iran and Russia in other countries around the world.
In a statement released on Wednesday, the FBI said that “Iranian malicious actors have continued their efforts since June to send stolen, non-public material associated with former President Trump’s campaign to US media organisations".
When contacted by the BBC, Trump campaign spokeswoman Karoline Leavitt said that the hack is proof that Iran was "interfering in the election to help Kamala Harris and Joe Biden because they know President Trump will restore his tough sanctions and stand against their reign of terror".
She said Biden and Harris should outline what happened to the material sent to Biden associates. "What did they know and when did they know it?” Ms Leavitt questioned.
Morgan Finkelstein, a spokesperson for the Harris-Walz campaign, said that the campaign has cooperated with authorities since it was made aware of the hacks.
"We're not aware of any material being sent directly to the campaign," Ms Finkelstein added. "A few individuals were targeted on their personal emails with what looked like a spam or phishing attempt."
Ms Finkelstein added that it condemns "in the strongest terms" any foreign election interference efforts.
The BBC has also contacted the White House for comment.
The FBI statement added that officials have been in contact with the victims of the hack and will continue to investigate in hopes of stopping and disrupting "the threat actors responsible”.
The announcement comes amid renewed scrutiny of Trump's security, just days after a second apparent assassination attempt was made against him at his golf course in Florida.
Speaking to reporters earlier, Trump addressed the attempt, saying that "I just have to lead my life".
"You don't want to ever be curtailed because of the crazy people out there," he added.
After Trump was wounded by a gunman during a 13 July rally in Butler, Pennsylvania, US officials said that an Iranian threat on Trump's life had prompted additional security measures.
At a packed rally in Long Island in New York on Wednesday - just a few miles away from where he grew up in Queens - Trump said that "God spared my life".
"Not once, but twice," he said to raucous applause from the crowd. "There are those that say he [God] did it because Trump is going to turn this state around. He's going to turn this country around."
Supporters at the rally told the BBC they were angry after learning about the second possible attempt after authorities say gunman was found with a rifle at Trump's Mar-a-Lago club in Florida as the former president was golfing.
One supporter at the rally, Dina Glazer, said she blamed Democrats and their rhetoric about Trump for the incidents.
The former president “needs more security, which they haven’t done”, she said.
Another supporter, Michelle Christ, said she feared Trump was in "constant danger".
“Some people think their opinion is the most important,” she said. “But you don’t act on those thoughts” violently, she said.
The crowd at the rally booed as Trump mentioned Springfield, Ohio, which has been rocked by unfounded claims about its Haitian immigrant community. Trump faced criticism for repeating them during the presidential debates.
“Send them back!” One member of the crowd shouted from the back of the arena.