The intelligence community inspector general, Michael Atkinson, wrote in a letter to the acting director of national intelligence, Joseph Maguire, that President Donald Trump's conduct could make him susceptible to blackmail by foreign intelligence services.

Atkinson was referring to Trump's repeated attempts to pressure Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden during a July 25 phone call.

Atkinson determined that in addition to potentially violating campaign finance laws and being a "flagrant" abuse of power, Trump's conduct could pose a major threat to US national security.

Specifically, Atkinson wrote that Trump's actions raise "counterintelligence risks with respect to foreign intelligence services aware of such alleged conduct."

Foreign spymasters could use the secrecy surrounding the effort to uncover dirt on the Bidens in Ukraine to feed disinformation or even blackmail Trump or Giuliani, a private citizen whom Maguire was unsure even had a security clearance.

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The intelligence community inspector general (IC IG) Michael Atkinson said in a letter to the acting director of national intelligence (DNI) Joseph Maguire that President Donald Trump's conduct during a July 25 phone call with Ukrainian President Volodymyr Zelensky is a serious threat to US national security.

A whistleblower complaint detailing the call that was submitted to Atkinson alleged that Trump repeatedly pressured Zelensky to investigate former Vice President Joe Biden and discredit the Russia investigation during their conversation.

Atkinson determined that in addition to potentially violating campaign finance laws, Trump's conduct could also open him up to foreign blackmail.

Specifically, he wrote to Maguire, "alleged conduct by a senior US public official to seek foreign assistance to interfere in or influence a federal election would constitute a 'serious or flagrant problem'" and would also "potentially expose such a US public official to serious national security and counterintelligence risks with respect to foreign intelligence services aware of such alleged conduct."

Read more: Trump suggested the whistleblower who filed a complaint against him is guilty of treason, which is punishable by death

The whistleblower's complaint, as well as Atkinson's letter to Maguire, were released on Thursday as Maguire testified before the House Intelligence Committee about the matter.

The complaint also alleged that Trump's personal lawyer Rudy Giuliani was a key figure in Trump's push and that Attorney General William Barr "appears to be involved as well." It said Trump urged Zelensky to "meet or speak with two people the President named explicitly as his personal envoys on these matters, Mr. Giuliani and Attorney General Barr, to whom the President referred multiple times in tandem."

Foreign spymasters could use the secrecy surrounding the effort to uncover dirt on the Bidens in Ukraine to feed disinformation or even blackmail Trump or Giuliani, a private citizen whom Maguire was unsure even had a security clearance.

Trump and his allies have sought to downplay the significance and accuracy of the complaint by saying it's based on hearsay and secondhand information.

But Maguire confirmed on Thursday that the complaint largely aligns with a memo of the July 25 phone call that the White House released on Wednesday.

And although the whistleblower said they were not a direct witness to the conduct described in the complaint or to Trump's conversation with Zelensky, they said they learned of it from "multiple White House officials with direct knowledge of the call."

Read more: Trump is facing the biggest firestorm of his presidency because his own White House staffers blew the whistle on him

The whistleblower said they heard "various facts related to" it from more than half a dozen US officials. "Multiple officials recounted fact patterns that were consistent with one another," the complaint said.

The complaint said White House officials who informed the whistleblower of Trump's call were "deeply disturbed" by what they had heard. They also told the whistleblower that White House lawyers were already discussing how to handle the call because they believed they'd witnessed Trump "abuse his office for personal gain."

On Tuesday, it surfaced that Maguire and the intelligence community inspector general, Michael Atkinson, referred the complaint to the Justice Department for criminal investigation.

But the Justice Department's criminal division reviewed the whistleblower's complaint and determined that there were no grounds for an investigation of Trump's behavior. Officials decided that the memo of Trump's phone call with Zelensky didn't show him violating campaign finance laws by asking for a financial contribution or an "item of tangible value."