American intelligence agencies announced on Monday that Iran was responsible for hacking into the campaigns of former President Donald J. Trump and attempting to breach the Biden-Harris campaign. The revelation came shortly after Roger J. Stone, a longtime Trump adviser, disclosed that his Hotmail and Gmail accounts had been compromised, allowing Iranian hackers to impersonate him and access campaign aides' emails. The announcement highlighted the growing foreign interference in the 2024 election amid increased domestic polarization and escalating tensions between Iran, Israel, and their international allies, including the United States. In a joint statement, intelligence officials stated that Iran aimed to sow discord and undermine confidence in democratic institutions, citing the Islamic Republic's history of exploiting societal tensions. Iran denied the allegations and called for evidence to support the claims. The U.S. intelligence agencies expressed confidence that Iran used "social engineering" to pose as trusted individuals within organizations' social networks to gain access to presidential campaign communications. The officials noted that Iran's government has become increasingly aggressive, viewing the election's outcome as crucial to its national security interests. While the specific outcome Iran might favor was not specified, both the Trump administration's withdrawal from the nuclear agreement and the tough stance taken by leading Democrats, including President Biden and Vice President Kamala Harris, were mentioned. The F.B.I. and the Cybersecurity and Infrastructure Security Agency advised email users to enhance their security measures and report any potential hacking attempts. The F.B.I. confirmed that it was investigating a campaign cyberintrusion, with indications that attempts were made to infiltrate accounts associated with the Democrats' presidential campaign. However, there was no indication that the efforts had succeeded, and the Biden-Harris campaign was not aware of any breaches to its systems. The joint statement did not provide new details about the attacks or specify how the agencies knew Iran was responsible. The U.S. intelligence agencies urged email users to strengthen their passwords, install software updates, avoid opening suspicious attachments, and report any possible hacking efforts to law enforcement officials.