Washington (CNN) A former US official who left the State Department in 2012 received a letter in August informing him that dozens of his emails sent to then-Secretary of State Hillary Clinton were now being recategorized as classified.

The letter marks one instance of what the Washington Post reported Saturday as a wide-scale reclassification of emails sent to Clinton's private email by as many as 130 current and former senior State Department officials.

The recent change in the investigation was an effort to harass diplomats for doing their job, several people told the Post, as Trump had made Clinton's use of private email servers during her tenure as secretary of state a staple of his 2016 presidential campaign rhetoric.

Trump weighed in on Clinton's emails again Sunday night, slamming her for "33,000 emails that she has deleted and acid washed so they can never be found."

The report of the intensifying email probe against Trump's former political rival comes days after the release of a whistleblower complaint that alleges Trump abused his official powers "to solicit interference" from Ukraine in the upcoming 2020 election, and that the White House took steps to cover it up. Trump has denied any wrongdoing. A transcript released by the White House shows Trump repeatedly pushed Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden, his potential 2020 political rival, and his son, Hunter Biden.

There is no evidence of wrongdoing by Joe or Hunter Biden.

House Democrats this past week launched a formal impeachment inquiry into the President.

The former official told CNN he first received a letter by mail from the Bureau of Diplomatic Security in January 2018, years after leaving the government. The letter alerted him that some of his emails sent on the unclassified system should have been sent on a classified system.

Then on August 5, another letter from the Bureau of Diplomatic Security was sent -- this time in an email -- informing him of the change in status of the emails.

"It has been determined that, at the time they were sent, a number of these emails contained classified information which were not property marked as such," the letter stated.

The official told CNN that it appeared to him the emails were reviewed again, but with a stricter criteria because more emails were flagged in the second letter that the agency said should have been labeled classified. The official said he has talked to multiple former and current State Department colleagues who received a new letter and that there is a lot of confusion among them about why this is happening now.

Current and former officials told the Post dozens of officials have been contacted in recent weeks by investigators at the State Department.

Those targeted, including senior officials as well as others in lower-level jobs, have been notified that emails they sent years ago have been retroactively classified and now are potential security violations, the newspaper said.

While a former senior US official familiar with the email investigation told the Post that the email investigation is a way for Republicans "to keep the Clinton email issue alive," senior State Department officials countered to the newspaper that they are following standard protocol in an investigation that began before Trump became President.

An FBI investigation found that of the 30,000 emails Clinton provided in 2014 to the State Department from her server, 110 contained classified information at the time they were sent or received. The agency did not recommend charges against Clinton, though then-FBI Director James Comey rebuked her and her aides for being "extremely careless."