The Federal Bureau of Investigation (FBI) has launched an investigation into a suspected Iranian cyber intrusion into the campaign of Republican presidential nominee Donald Trump, which may have resulted in the theft of sensitive internal documents. The investigation also includes attempts to infiltrate President Joe Biden's reelection campaign.

Trump's presidential campaign revealed on Saturday that it had been hacked and that sensitive internal documents were stolen and distributed. The campaign accused Iranian actors of being responsible for the hack. The same day, Politico reported receiving leaked internal Trump campaign documents via email from an individual identified only as "Robert." The leaked materials included vetting materials on Republican vice presidential nominee JD Vance and Sen. Marco Rubio, who was also considered for the vice president position.

Two other news outlets, The New York Times and The Washington Post, also reported receiving leaked materials related to the Trump campaign. However, they did not disclose specific details about the contents of the leaked documents. It remains unclear whether these leaked materials are connected to the alleged hacking of the Trump campaign.

Trump's campaign spokesperson, Steven Cheung, suggested that there was a connection between the leaked materials and the alleged hacking incident. He warned that any media or news outlet reprinting the leaked documents or internal communications were doing the bidding of America's enemies and fulfilling their objectives.

The FBI confirmed on Monday that it was indeed investigating the reported intrusion into the Trump campaign's systems. Two individuals familiar with the matter also revealed that the FBI was looking into attempts to gain access to Biden-Harris' campaign.

Trump's campaign did not provide specific evidence pointing to Iran's involvement in the hacking incident. However, they pointed to a Microsoft report released on Friday that detailed an Iranian attempt to infiltrate a presidential campaign in June. The report mentioned an Iranian military intelligence unit sending a spear-phishing email to a high-ranking official of a presidential campaign from a compromised email account of a former senior advisor. Spear-phishing is a type of cyberattack where an attacker poses as a known or trusted sender to install malware or gather sensitive information.

Microsoft did not disclose which campaign or advisor was targeted but confirmed that they had notified the affected party. Both Trump and his longtime friend and advisor, Roger Stone, have since revealed that they were contacted by Microsoft regarding potential cyber intrusions.

Iran's mission to the United Nations denied any involvement in the alleged hacking incident, stating that they do not accord any credence to such reports and do not possess any intent or motive to interfere in the United States' presidential election. However, Iran has been previously suspected of conducting hacking campaigns targeting its enemies in the Middle East and beyond. Tehran has also threatened retaliation against Trump for ordering the 2020 drone strike that killed prominent Revolutionary Guard Gen. Qassem Soleimani.

Harris' campaign declined to comment on whether they had identified any state-based intrusion attempts but emphasized their vigilance in monitoring cyber threats and their lack of knowledge about any security breaches of their systems. However, two individuals familiar with the investigation revealed that the Biden-Harris campaign was also targeted in the suspected Iranian cyber intrusion. At least three staffers in the Biden-Harris campaign were targeted with phishing emails, but investigators have found no evidence of successful intrusions. The FBI began investigating this cyber incident in June, with intelligence officials believing Iran was behind the attempts.

This incident bears similarities to the 2016 hack that exposed emails to and from Hillary Clinton's campaign manager, John Podesta. The website Wikileaks published a trove of these messages, which were extensively reported on by news outlets. Senate Majority Leader Chuck Schumer highlighted the repeated use of such tactics against the United States and warned that foreign adversaries are intent on sowing chaos and undermining the democratic process.