The transcripts and notes of President Donald Trump’s conversations with foreign government officials that administration officials restricted access to are Trump’s version of President Richard Nixon’s secret White House tapes: a trove of potentially deeply damaging information that the lawmakers moving to impeach the president are eager to get their hands on. Already, the release of one of these transcripts — the summary of Trump’s call with Ukrainian President Volodymyr Zelensky — revealed the president using his office and the international position of the United States to pressure a foreign country to open investigations to help his own reelection campaign. White House officials moved the Trump-Zelensky call summary to a “standalone computer system reserved for codeword-level intelligence information, such as covert action,” according to the Aug. 12 whistleblower complaint. “[T]his was ‘not the first time’ under this Administration that a Presidential transcript was placed into this codeword-level system solely for the purpose of protecting politically sensitive ― rather than national security sensitive ― information,” the complaint states. Although no specific conversations have been alleged to be improperly placed on the highly classified server, White House officials heavily restricted access to or failed to file notes on talks Trump held with Russian President Vladimir Putin, Saudi Arabia Crown Prince Mohammed bin Salman and Russian diplomats. The contents of phone calls Trump held with Putin and bin Salman were heavily restricted within the White House, according to CNN. No transcript or summary was made of a call with bin Salman which occurred in the immediate aftermath of Saudi Arabia’s murder of journalist Jamal Khashoggi in Istanbul, according to the CNN report. The New York Times later reported that the transcripts of Trump’s calls with bin Salman, Saudi King Salman and Prince Khalid bin Salman were placed in the same restricted system as the Ukraine call.

Anadolu Agency via Getty Images Calls between President Donald Trump and Saudi Crown Prince Mohammed bin Salman were restricted by the White House.

Trump also reportedly told Russian officials that he didn’t care about their interference in the 2016 election at an Oval Office meeting the day after he fired FBI director James Comey, according to The Washington Post. The notes memorializing this meeting were “restricted to a very small number of people.” Leaders of the impeachment inquiry into Trump’s actions toward Ukraine are now eyeing other Trump conversations that were placed on the same server as the Ukraine call or otherwise restricted to protect politically sensitive conversations. “If those conversations with Putin or with other world leaders are sequestered in that same electronic file that is meant for covert action, not meant for this, if there’s an effort to hide those and cover those up, yes, we’re determined to find out,” House Intelligence Committee Chairman Adam Schiff (D-Calif.) said on NBC’s “Meet the Press” on Sunday. It is not known how many of these call transcripts were placed on the same codeword server as the Zelensky call. Nor is it known if those calls contained anything sensitive to national security or, like the Ukraine call, just politically sensitive information. But the White House has reportedly moved other, as yet unknown calls, to the server. The White House placed increased internal restrictions on access to transcripts of Trump’s talks with foreign leaders after calls with the leaders of Australia and Mexico leaked to the press. A spokesman for Putin declared on Monday that the White House should not release transcripts of the Russian leaders’ calls with Trump without Russian permission. The Nixon tapes contained the “smoking gun” evidence that the president asked his aides to get the CIA to shut down the FBI’s probe into the break-in of the Democratic National Committee offices at the Watergate hotel. Their existence was revealed to Congress by the White House aide who installed the taping system in the Oval Office in 1971. The tapes ultimately confirmed testimony by former White House counsel John Dean that Nixon sought to cover up the Watergate break-in.

Bettmann via Getty Images President Nixon in August 1973, conducting the first press conference in nine months, answers one of the first questions asked regarding his withholding of the tapes involved in the Watergate affair.