The FBI's investigation into Rudy Giuliani, President Donald Trump's lawyer, includes a previously unknown counterintelligence aspect that could alter the entire nature of the probe.

CNN reported that investigators approached a lawyer named Kevin McCallion earlier this year to ask him about Giuliani's ties to two Soviet-born men who were arrested and charged last week with breaking campaign-finance law and trying to funnel foreign money into US political campaigns.

Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department, told Insider the existence of a counterintelligence probe means investigators are "casting a wide net."

Asha Rangappa, a former FBI special agent, also noted that the counterintelligence investigation indicates that the FBI is concerned Giuliani poses a national security threat.

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The FBI's investigation into Rudy Giuliani, President Donald Trump's personal lawyer, includes a previously unknown counterintelligence component that could change the entire nature of the probe.

CNN reported on Wednesday that investigators from the Manhattan US attorney's office approached Kevin McCallion, an attorney in New York, earlier this year to ask about Giuliani's link to Lev Parnas and Igor Fruman, two Soviet-born men who were arrested and charged last week with violating campaign finance laws and trying to funnel foreign money into US political campaigns.

Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department, told Insider the existence of a counterintelligence probe means investigators are "casting a wide net."

"Cases like these, and they aren't common, usually involve multiple schemes and numerous players," Cramer said. "As such, they touch upon many different possible criminal violations."

McCallion told CNN that counterintelligence investigators from the FBI also asked him in February or March about Giuliani's business dealings in Ukraine.

"I was just asked whether I or any of my clients knew of any dealings that these two guys had with Giuliani," McCallion told CNN. "They were on the radar with regard to possible counterintelligence issues."

The counterintelligence investigation is reportedly focused on whether a foreign influence operation tried to use Giuliani's financial interests in Ukraine as a conduit to the White House.

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Giuliani has come under scrutiny for his ongoing push to get Ukraine's government to investigate former Vice President Joe Biden and his son for corruption. Parnas and Fruman were both part of Giuliani's efforts in the matter.

The former New York mayor is also under criminal investigation — separate from the counterintelligence probe — for his role in the abrupt ouster of Marie Yovanovitch, the US's former ambassador to Ukraine, and whether he broke foreign lobbying laws in the process.

Specifically, prosecutors are said to be examining whether Giuliani was working on behalf of the Ukrainian prosecutor general, Yuriy Lutsenko, while pushing for Yovanovitch's dismissal.

The Southern District of New York — nicknamed the "Sovereign District of New York" — has a longstanding reputation for operating largely independent from the Justice Department's headquarters in Washington, DC. CNN's report said the office's investigation into Giuliani and his associates goes much farther back than what has previously been reported.

The investigation is said to be looking in part at who financed Giuliani's work in Ukraine, which included collaborating with Parnas and Fruman and lobbying the Ukrainian government on behalf of Kharkiv, a small town located in eastern Ukraine.

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Asha Rangappa, a former FBI special agent, wrote on Twitter that the existence of a counterintelligence investigation into Giuliani indicates that the FBI believes "he may pose a national security threat to the United States."

Such investigations aren't rooted in suspicions that someone violated US laws, but rather in concerns that the individual was in contact with people linked to foreign intelligence, or that they're working to further the interests of a foreign power in the US, Rangappa added.

The bar for opening this type of investigation into Giuliani, who is a US national and the president's personal lawyer, is also higher than it is for probing foreign individuals.

"So basically, the FBI thinks something bad — and likely not 'unwitting' — is up," Rangappa wrote. "That Giuliani is a conduit for pushing the agendas of foreign intelligence and/or foreign interests."

Although criminal investigations and counterintelligence investigations typically run on two separate tracks, Rangappa noted that it's possible a counterintelligence probe can include criminal violations. It's unclear what those would be in Giuliani's case, but previous reporting suggests the former New York mayor could be accused of violating the Foreign Agents Registration Act (FARA), which governs foreign lobbying in the US.

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In addition to his work in Ukraine, Giuliani also raised red flags among White House aides by repeatedly pushing Trump to extradite Fethullah Gulen, a controversial Muslim cleric, to the Turkish government, the Washington Post reported this week.

Gulen lives in exile in Pennsylvania and has been accused by Turkish President Recep Tayyip Erdogan of orchestrating a failed 2016 coup against him and of generally fomenting dissent within Turkey. Gulen denies any involvement in the attempted coup.

Getting the US to extradite Gulen has long been a top priority for Erdogan, and Giuliani's reported efforts to facilitate that goal raises further questions about the extent to which he tried to influence government policy while acting as a private citizen.

The Post reported that Giuliani mentioned Gulen so often that one former official described the topic as the former New York mayor's "hobby horse."

"It was all Gulen," a second former official told the Post. Giuliani was so intent on the matter that some White House aides reportedly worried he was working on behalf of the Turkish government.