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WASHINGTON — The inspector general for the Justice Department is probing the classification status of information in the memos written by James Comey that became public Thursday after they were provided to Congress, a source familiar with the investigation told NBC News.

As first reported by The Wall Street Journal, the DOJ's internal watchdog is interested in two documents, one that contained information that was not classified when Comey was at the FBI, but has since been upgraded to "confidential," and another that Comey knew contained classified information but that he had redacted before he gave the memo to a friend.

Confidential is the lowest category of U.S. government classification, less sensitive than "secret" and "top secret."

Comey gave four memos to his friend Daniel Richman, and has said he considers the documents to be personal. Richman, a former federal prosecutor, is a professor at Columbia Law School, where Comey worked briefly in 2013. Richman told NBC News in 2017 that the men have known each other for 30 years.

Comey said in June 2017 that he had given the memos to his "good friend" Richman so they could be made public.

President Donald Trump tweeted late Friday about the news.

James Comey illegally leaked classified documents to the press in order to generate a Special Council? Therefore, the Special Council was established based on an illegal act? Really, does everybody know what that means? — Donald J. Trump (@realDonaldTrump) April 21, 2018

Trump fired Comey as FBI director in May 2017.