American intelligence agencies said on Monday that Iran was responsible for hacking into former President Donald J. Trump’s campaign and trying to breach the Biden-Harris campaign.
The finding, which was widely expected, came days after a longtime Trump adviser, Roger J. Stone, revealed that his Hotmail and Gmail accounts had been compromised. That intrusion evidently allowed Iranian hackers to impersonate him and gain access to the emails of campaign aides.
The announcement was the starkest indication to date that foreign intelligence organizations have mobilized to interfere in the 2024 election at a moment of heightened partisan polarization at home and escalating tensions abroad between Iran and Israel, along with its international allies, including the United States.
“Iran seeks to stoke discord and undermine confidence in our democratic institutions,” intelligence officials wrote in a joint statement from the F.B.I., the Office of the Director of National Intelligence and the Cybersecurity and Infrastructure Security Agency.
The Islamic Republic has “demonstrated a longstanding interest in exploiting societal tensions through various means,” the officials added.
The joint statement provided no new details about the attacks, nor did it specify how the agencies knew Iran was responsible.
Iran denied that it was behind the hacking, calling the allegations “unsubstantiated and devoid of any standing.”
“The Islamic Republic of Iran harbors neither the intention nor the motive to interfere with the U.S. presidential election,” Iran’s mission to the United Nations said in a statement. “Should the U.S. government genuinely believe in the validity of its claims, it should furnish us with the pertinent evidence — if any — to which we will respond accordingly.”
But the U.S. intelligence agencies expressed confidence that the Iranians used “social engineering,” posing as trusted members of an organization’s social network to people with direct access to the inner communications of the presidential campaigns in both parties.
Iran’s government has shown signs of becoming increasingly aggressive in recent months, because it sees the outcome of the elections as “particularly consequential in terms of the impact they could have on its national security interests,” the officials said.
They did not specify which, if any, outcome Iran might favor. In 2018, Mr. Trump scrapped an agreement reached in the waning days of the Obama administration to limit and monitor Tehran’s civilian nuclear development program in exchange for easing economic sanctions.
But leading Democrats, including President Biden and Vice President Kamala Harris, have also taken a tough line with Iran, the chief sponsor of Hezbollah and Hamas.
The F.B.I. and the Cybersecurity and Infrastructure Security Agency urged email users to strengthen their passwords, install software updates, avoid opening suspicious attachments and alert law enforcement officials of any possible hacking efforts.
A week ago, the F.B.I. confirmed that it was investigating “a campaign cyberintrusion,” days after Mr. Trump asserted that Iran had targeted his campaign. The bureau did not specifically name Iran or Mr. Trump at the time — nor did it indicate the extent of the breach or the possibility that it encompassed other campaigns or political figures.
But investigators appeared to also be examining an attempt to infiltrate accounts associated with the Democrats’ presidential campaign, according to a law enforcement official with knowledge of the situation.
The timing of the attempt was unclear, though the official added that there was no indication that the effort had succeeded. Ms. Harris’s team, which carefully monitors cyberthreats, is not aware of any breach to its systems, according to a campaign official who spoke on the condition of anonymity to discuss security arrangements.
