Washington (CNN) Secretary of State Mike Pompeo has become increasingly frustrated in recent weeks by the departure of top State Department officials and claims that he failed to defend the former US Ambassador to Ukraine, Marie Yovanovitch, from a smear campaign against her, according to three sources familiar with the situation.

One of the sources tell CNN that Pompeo was alerted to internal and external concerns about Giuliani's effort to push out Yovanovitch, but Pompeo failed to act -- he was wary of getting too deeply involved over fears of derailing US-Ukraine policy and potentially sharing the fate of his former colleague John Bolton, Trump's national security adviser who was fired for not being aligned with the President.

In a letter sent in the spring, which has not been previously reported, a handful of former US ambassadors to Ukraine urged top State Department officials to take action and defend Yovanovitch. They got a response from one of Pompeo's closest senior officials, Counselor Ulrich Brechbuhl, saying the message had been received and would be considered.

Yet Pompeo did nothing and less than two months later, Yovanovitch was recalled from her post at the behest of Trump.

Pompeo's apparent choice not to put guardrails between State officials and Giuliani has come under scrutiny by House Democrats who are bent on learning as much as they can about what exactly Pompeo knew of Giuliani's dealings. In recent interviews, Pompeo has declined to answer questions about Giuliani's entanglement in the administration's Ukraine policy.

He was however aware of it. In March, Pompeo received a packet from Giuliani containing unfounded claims about Yovanovitch, as well as former Vice President Joe Biden and his son Hunter regarding their dealings in Ukraine.

Giuliani has claimed Pompeo spoke with him to confirm the packet had been received and that he was passing along the information for an investigation. It is unclear if Pompeo actually did share the packet with the Department of Justice. Pompeo's deputy, Brechbuhl, did pass it to the State Department's inspector general.

Pompeo has had to accept the resignation of two top State Department officials, Kurt Volker, the former special representative to Ukraine, who resigned last month , and Michael McKinley, his hand-picked senior adviser, who quit last week . Both have testified to Congress despite the State Department's refusal to comply with subpoena requests.

The State Department did not return CNN requests for comment.

Mounting criticism about his handling of the matter has left the Secretary of State feeling victimized, according to those familiar with his thinking.

"He has been very disappointed," explained one of the sources who spoke with Pompeo. "He feels that with these departures, the actual good work on Ukraine policy has come to a halt."

Keeping his distance

US Secretary of State Mike Pompeo has faced criticism for not defending former US Ambassador to Ukraine, Marie Yovanovitch.

As a string of State Department officials have testified on Capitol Hill , Pompeo has stayed away from Washington and kept a packed foreign schedule. He was visiting with the Pope in Italy when Volker voluntarily testified to Congress. And this week he traveled to Turkey for talks on Syria with Vice President Mike Pence as even more State Department officials testified.

Despite his efforts to distance himself, the continued testimony from current State Department officials has drawn Pompeo closer into the controversy.

In his prepared statement on Thursday, US Ambassador to the European Union Gordon Sondland aligned himself with career diplomats and passed responsibility for his role in the situation to political players, including Trump and Pompeo.

Sondland said that Pompeo was aware of all his efforts on Ukraine policy and set him to work on the country from his first day on the job, despite the fact that Ukraine is not a member country of the EU. Sondland said Pompeo even sent him a recent congratulatory note, saying that he was "doing great work," and to "keep banging away."

Pompeo has refused to turn over documents subpoenaed by House Democrats as part of their impeachment inquiry.

Former State Department senior adviser McKinley also testified Wednesday that he repeatedly asked Pompeo for a show of support for Yovanovitch -- and as Pompeo remained silent, he never explained his reasoning.

McKinley had grown close with Pompeo over the last two years and was recruited into the top post by the secretary. McKinley told lawmakers that Pompeo's decision not to offer Yovanovitch support was a key reason for his decision to resign last week.

However, a source close to Pompeo pushed back on the way that McKinley told the story, telling CNN, "I think (McKinley) knew damn well that Pompeo was trying to help her."

Despite what he may have been doing behind the scenes, Pompeo ultimately failed to intervene or speak out publicly on Yovanovitch's behalf before or after she was recalled.

In fact, it seems Pompeo attempted to distance himself even further from the controversy in the heat of the moment. He did not tell Yovanovitch exactly what was happening when she got the sudden message to get on the next plane back to the US, she told lawmakers.

Yovanovitch told Congress that upon her return to Washington it was John Sullivan, the Deputy Secretary of State at the time, who met with her to break the news she would not be going back to Ukraine as the US ambassador.

Some former State Department officials felt Pompeo was somewhat justified in not publicly defending Yovanovitch.

"She was treated badly but she could have been treated worse. She could have been fired and sacked from the State Department but that did not happen," John Herbst, a former US Ambassador to Ukraine and expert at the Atlantic Council, told CNN.

"I understand foreign service officers who wanted him to shout from a rooftop that the accusations against her were unfounded and she was a great Ambassador. I would have liked to see that. But maybe he was not sure that he could do that and retain his job or influence," he said.

Broader implications

Critics argue that by not saying anything to defend Yovanovitch, Pompeo has sided with a foreign disinformation campaign built on false narratives dealing with Ukraine amplified by Giuliani, Trump and sources of Russian propaganda.

"The lack of support has broader implications," said Steve Pfifer, a former US ambassador to Ukraine under President Bill Clinton, and currently a senior fellow at the Brookings Institution. Pompeo's decision not to defend Yovanovitch, intentionally or not, sent a message to the world, Pfifer said. "The word is that if you have an American ambassador and he or she is problematic, put out some information, get it to Rudy Giuliani that this ambassador bad-mouthed the President."

While CNN's fact-checkers have largely discredited the unfounded claims about former Vice President Joe Biden and his son, Hunter, related to their dealings with Ukraine, it's challenging to determine their origin. It seems that Trump, Giuliani and Kremlin-backed media have engaged in parallel processes of using this information as part of a broader strategy of denying everything by blaming anyone else, said Bret Schafer from Alliance for Securing Democracy.

whistleblower complaint regarding Trump's dealings with Ukraine, which was released publicly last month, alleged Trump "is using the power of his office to solicit interference from a foreign country in the 2020 U.S. election" and labeled Giuliani as a "central figure" in the plot.

The transcript of Trump's July 25 phone with Ukrainian President Volodymyr Zelensky -- that was also released to the public -- showed Trump repeatedly pushed for Zelensky to investigate Joe Biden and Hunter Biden. Giuliani was discussed, according to the transcript, with Trump telling the Ukrainian president at one point, "I will have Mr. Giuliani give you a call."

Pompeo admitted, about a week after the phone call's transcript was released, that he was on that call. And in recent interviews he has repeatedly defended Trump's conversation as entirely appropriate.

George Kent, a State Department official who focused on Ukraine, told lawmakers Tuesday that his superior, assistant secretary for European Affairs Philip Reeker, told him to "lay low" when he raised concerns about Giuliani.

Now, Congress has requested to hear from Reeker.

It is unclear if Pompeo told top State Department officials not to raise concerns about Giuliani or the removal of Yovanovitch.

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The testimony from McKinley this week showed Pompeo has kept a tight hold on his thinking throughout this controversy, even with those he is closely engaged with.

Meanwhile, in the last few weeks, the State Department has fallen in line with the White House in not complying in part with the impeachment inquiry.

Ambassador Sondland's lawyer explained in a letter to congressional committees that Sondland and others in his position have been prevented from providing documents to Congress because the State Department "asserted that disclosure of these materials may implicate executive privilege, confidentiality, and other constitutional interests of the executive branch."

The failure to release pertinent documents to Congress raises questions about what details they provide regarding Pompeo's awareness and actions on the specific developments.

As the State Department has stopped working with Congress due to orders from the White House, Pompeo himself continues to defend Trump. He has called the Ukraine impeachment inquiry a conspiracy theory to take down the President.

But there will be new questions about conversations regarding quid pro quo in the coming days given the pronouncement by acting White House Chief of Staff Mick Mulvaney in a news conference Thursday that the US withheld military aid from Ukraine in hopes of securing a promise that the foreign government would look into the possible presence of the physical Democratic National Committee server hacked by the Russians in 2016.

Pompeo, whose department oversees a hefty amount of that assistance to Ukraine, was also asked last month if it would be a problem if there were a quid pro quo arrangement. Pompeo said it is his job to be sure that foreign assistance is "completely appropriate" and that "American taxpayer dollars are used appropriately."