Interviews with intelligence community officials have revealed that senior FBI leaders "seemed indifferent to evidence of a possible intrusion by a foreign adversary" into former Secretary of State Hillary Clinton’s non-government email server, and that State Department officials allegedly sought to "downgrade classified material found on the server," according to Senate investigators still probing the matter.

The information was contained in a letter and interview transcripts sent Monday by the majority staff on the Senate Finance and Homeland Security and Governmental Affairs Committees to senior Senate Republicans including Finance Committee Chairman Chuck Grassley, R-Iowa. The letter also noted that “neither the committees nor the FBI were able to confirm whether an intrusion into the server occurred."

The review began after two August 2018 news articles "alleged that a Chinese state-owned company hacked former Secretary Clinton’s nongovernment server and inserted code that forwarded nearly all of her emails to the foreign company," the letter stated, citing an article in The Daily Caller, and a later piece by Fox News.

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A source briefed on the matter confirmed to Fox News at the time that the Office of the Intelligence Community Inspector General (ICIG) was so concerned by the revelation that officials drove over to the FBI to inform agents -- including anti-Trump agent Peter Strzok -- of the development soon after it was discovered via the emails' metadata in late 2015 or early 2016.

The source said the hack was from a Chinese company, describing it as a front for Chinese intelligence.

The letter to Grassley continued: "The reporting indicated that two Intelligence Community Inspector General [ICIG] officials – Frank Rucker and Jeanette McMillian – discovered the code and brought the possible intrusion to the attention of the FBI."

In a December 2018 interview with Senate investigators concerning the reporting on the potential foreign hack, Rucker said Strzok -- then the deputy assistant director of the FBI’s counterintelligence division in charge of the Clinton investigation -- seemed "aloof and dismissive."

And Strzok didn't ask many questions, according to Rucker.

McMillian, meanwhile, said FBI employees were "poker-faced" during the roughly hourlong meeting about the potential breach, even though both the CIA and the National Geospatial-Intelligence Agency (NGA) had "significant equities" in the information contained on the server.

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The FBI did not immediately respond to Fox News' request for comment.

McMillian told investigators the episode began when Rucker identified an email address in Clinton's email metadata as a potential "live dropbox" for all her emails, which a foreign actor could access.

When Rucker then Googled the email address on the suggestion of then-ICIG Charles McCullough, the name of a Chinese company, "Shandong Carter Heavy Industry Machinery," popped up -- leading Rucker and McCullough to suspect Chinese involvement, according to Rucker's interview with investigators.

In April, the ICIG and Justice Department IG determined that the address apparently matched the one that former Clinton technology aide Paul Combetta -- who managed her server and thoroughly deleted a slew of her emails -- had created as a "dummy" to create backups of Clinton's emails.

However, the DOJ IG and ICIG said they “did not identify any information obtained by the FBI during the [Clinton email] Midyear investigation reflecting that Combetta’s dummy email account was associated with a Chinese company or the Chinese government."

Throughout, according to McMillian's remarks to Senate investigators, FBI brass generally seemed "impressed" with Rucker's technical abilities in finding the apparent breach, and apparently took the issue seriously. McMillian specifically said Strzok had found the email account's presence in Clinton's metadata "strange."

McMillian said she made sure to tell the FBI that the ICIG was not trying to usurp the FBI's authority after bureau officials asked how it had found information about the potential breach. The FBI made clear the ICIG would provide information to the bureau, but not the other way around, McMillian and Rucker said.

A May 2016 email from Strzok, obtained by Fox News last year, said “we know foreign actors obtained access” to some Clinton emails, including at least one “secret” message “via compromises of the private email accounts” of Clinton staffers.

Strzok, earlier this month, filed a lawsuit against the bureau and the Justice Department – arguing that the FBI caved to the “unrelenting pressure” of the president when it fired him.

DOJ IG Michael Horowitz previously concluded that although he could not prove Strzok had let his political bias influence official decisions during the Clinton email investigation, he did not have confidence that Strzok had acted without bias at critical junctures in the probe.

In text messages with his then-lover, FBI lawyer Lisa Page, Strzok said on his government-issued phone that "we'll stop" Trump from becoming president, and called Trump supporters "retarded."

Separately, Rucker and McMillian offered more details into reports that the State Department sought to cover for Clinton by downgrading the classification of emails on the server.

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“At first, State fought back against the intelligence community being involved," McMillian told investigators, before noting that they eventually relented.

McMillian claimed that two State Department employees tried to shield some of Clinton's emails from disclosure under the "deliberative process" exemption under the Freedom of Information Act -- rather than labeling the emails classified.

The employees, Austin Evers and Catherine Duval, did not immediately return Fox News' request for comment. Evers has since launched the organization American Oversight along with other former Obama administration officials. The group says it is devoted to uncovering ethics issues in the Trump administration.

Rucker also claimed that then-Ambassador Patrick Kennedy questioned "whether certain emails should be provided to the intelligence community for review and fighting against classifying other emails," according to the Senate investigators.

The State Department didn't respond to Fox News' request for comment.

Interviews released in 2016, known as 302s, first revealed the serious allegation that Kennedy applied pressure to subordinates to change classified email codes so they would be shielded from Congress and the public.

Fox News was told as far back as August 2015 that Kennedy was running interference on Capitol Hill. But Kennedy, in his FBI interview on Dec. 21, 2015, “categorically rejected” allegations of classified code tampering.

“In return for altering the classification, the possibility of additional slots for the FBI at missions overseas was discussed,” then-House Oversight Committee Chairman Jason Chaffetz, now a Fox News contributor, said at the time.

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In a lengthy statement, the FBI denied it had agreed to declassify any emails in exchange for more overseas slots from the State Department.

A comprehensive IG report into potential FBI and DOJ misconduct during the Clinton and Russia probes is expected soon.

Fox News' Adam Shaw and Catherine Herridge contributed to this report.