Senate Homeland Security and Governmental Affairs Committee Chairman Ron Johnson on Thursday formally sought "all email communications" between Hillary Clinton and former President Obama, saying the Justice Department was blocking their release -- even though they could shed light on whether the former secretary of state discussed sensitive matters on her unsecured personal email system while she was overseas.

Johnson's letter came as House Democrats approved procedures for their impeachment inquiry against President Trump, warning he may have endangered U.S. national security by allegedly withholding aid to Ukraine for political reasons. Earlier this month, a State Department report into Clinton's use of a private email server for government business found dozens of people at fault and hundreds of security violations.

In a letter to the U.S. National Archives and Records Administration, Johnson, R-Wis., said summer 2016 communications from FBI Special Agent Peter Strzok to FBI Director James Comey’s Chief of Staff James Rybicki hinted at the existence of the Clinton-Obama messages that were relevant to the issues raised by her private server.

READ JOHNSON'S FULL LETTER

Johnson noted that on June 28, 2016, a week before Comey’s public statement declaring that "no reasonable prosecutor" would charge Clinton, Strzok wrote, "Jim – I have the POTUS – HRC emails [Director Comey] requested at end of briefing yesterday. I hesitate to leave them, please let me know a convenient time to drop them off."

"I write to request email communications between former Secretary of State Hillary Clinton and President Obama," Johnson wrote, setting a deadline of Nov. 14, 2019. "In January 2018, I requested the Department of Justice (DOJ) produce emails Secretary Clinton sent to President Obama while she was located in the 'territory of a sophisticated adversary.'"

He added: "Given that DOJ acknowledged that they 'are not in a position' to produce emails to the committee that contain 'equities of other executive branch entities,' I ask that, pursuant to the Presidential Records Act, you please provide all email communications between Secretary Clinton and President Obama."

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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. However, last year, the DOJ watchdog slammed Comey for speculating publicly that Clinton's emails had been hacked by foreign actors.

Interviews with intelligence community officials released this past August indicated that senior FBI leaders "seemed indifferent to evidence of a possible intrusion by a foreign adversary" into 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 probing the matter.

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The information was contained in a letter and interview transcripts sent by the majority staff on Johnson's Homeland Security Committee 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."

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Many Clinton emails already have been released. A batch of unearthed, heavily redacted and classified emails from Clinton's personal email server, published this past March, revealed that the former secretary of state discussed establishing a "private, 100% off-the-record" back channel to Israeli Prime Minister Benjamin Netanyahu, and that one of her top aides warned her she was in "danger" of being "savaged by Jewish organizations, in the Jewish press and among the phalanx of neoconservative media" as a result of political machinations by "Bibi and the Jewish leadership."

The 756-page group of documents, revealed as part of a transparency lawsuit by Judicial Watch, seemingly contradicted Clinton's insistence under oath in 2015 that she had turned over all of her sensitive work-related emails to the State Department, and included a slew of classified communications on everything from foreign policy to State Department personnel matters.

The files came from a trove of 72,000 documents the FBI recovered and turned over to the State Department in 2017.

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Additionally, according to the email dump, Clinton chatted with former U.K. Prime Minister Tony Blair about classified foreign policy matters before she was sworn in, aided the application of at least one State Department applicant who was connected to her daughter, Chelsea, and apparently met with Putin-aligned Georgian oligarch Bidzina Ivanishvili before he became prime minister on a staunchly pro-Russian platform -- and with reported help from a Russian interference operation.

Ivanishvili notably did not criticize Putin during his campaign, despite Putin's invasion of Georgia years earlier -- and in 2012, Ivanishvili made headlines for refusing to meet with Clinton unless it was a one-on-one sitdown.

Fox News' Jason Donner contributed to this report.