The FBI said Monday it is investigating allegations that sensitive documents from Donald Trump's presidential campaign were stolen in a cyber intrusion days after the campaign declared it had been hacked by Iran.

The FBI released a brief statement reading: "We can confirm the FBI is investigating this matter."

Two people familiar with the matter said the Biden-Harris campaign was also targeted in the suspected Iranian cyber intrusion that is under FBI investigation. They spoke on the condition of anonymity because they were not authorized to discuss the details of the investigation.

The Trump campaign provided no specific evidence of Iran's involvement, but the claim came shortly after Microsoft issued a report detailing foreign agents' attempts to interfere in the U.S. election in 2024. The report cited an instance of an Iranian military intelligence unit in June sending "a spear-phishing email to a high-ranking official of a presidential campaign from a compromised email account of a former senior advisor."

Politico reported Saturday that it began receiving emails on July 22 from an anonymous account. The source — an AOL email account identified only as "Robert" — passed along what appeared to be a research dossier the campaign had apparently done on the Republican vice-presidential nominee, Ohio Sen. JD Vance. The document was dated Feb. 23, almost five months before Trump selected Vance as his running mate.

Vice-President Harris's campaign said in a statement, "Our campaign vigilantly monitors and protects against cyber threats, and we are not aware of any security breaches of our systems." It declined to address whether it had identified any state-based intrusion attempts.

Iran's mission to the United Nations, when asked about the claim of the Trump campaign, denied being involved.

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However, Iran long has been suspected of running hacking campaigns targeting its enemies in the Middle East and beyond. Tehran also long has threatened to retaliate against Trump over the 2020 drone strike he ordered that killed prominent Revolutionary Guard Gen. Qassem Soleimani.

In its report, Microsoft stated that "foreign malign influence concerning the 2024 U.S. election started off slowly but has steadily picked up pace over the last six months due initially to Russian operations, but more recently from Iranian activity."

The analysis continued: "Iranian cyber-enabled influence operations have been a consistent feature of at least the last three U.S. election cycles. Iran's operations have been notable and distinguishable from Russian campaigns for appearing later in the election season and employing cyberattacks more geared toward election conduct than swaying voters."

"Recent activity suggests the Iranian regime — along with the Kremlin — may be equally engaged in election 2024," Microsoft concluded.

Contrast to 2016 campaign hack

Politico is one of at least three news outlets, along with the Washington Post and New York Times, that were leaked confidential material from inside the Trump campaign, including its report vetting Vance. So far, each has refused to reveal any details about what they received.

Their decisions not to publish stand in marked contrast to the 2016 presidential campaign, when a Russian hack exposed emails to and from Hillary Clinton's campaign manager, John Podesta. The website Wikileaks published a trove of these embarrassing missives, and mainstream news organizations covered them avidly.

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The Trump campaign was also eager to take advantage of any damaging information eight years ago.

"Russia, if you're listening," Trump famously said during a press conference in his 2016 presidential run, when Clinton's deleted personal emails while secretary of state were a hot topic. "I hope you are able to find the 30,000 emails that are missing ... I think you will probably be rewarded mightily by our press."

That changed when Trump's latest presidential campaign declared this weekend it had been hacked by Iran. 

"Any media or news outlet reprinting documents or internal communications are doing the bidding of America's enemies and doing exactly what they want," Steven Cheung, Trump campaign communications director, said in a statement on Saturday.

The campaign has not responded to questions about whether its view on hacking changed, including a query on Monday from The Associated Press.