James Comey, former director of the Federal Bureau of Investigation (FBI), listens during a Senate Intelligence Committee hearing in Washington, D.C., on Thursday, June 8, 2017.

Memos leaked by former FBI Director James Comey contain information that is now considered classified, prompting the Justice Department's watchdog to review the matter, the Wall Street Journal reported Friday, citing people familiar with the matter.

At least two of the memos Comey gave to a friend contained classified information, the Journal reported. Comey reportedly redacted portions of one of those two memos himself before sending them to his friend.

Comey wrote the memos after his interactions with President Donald Trump, recounting in detail his conversations with the president before he was fired last May. The memos, which were given to authorities last year, are now considered to be evidence in a potential obstruction of justice case against Trump, multiple outlets have reported.

The Office of the Inspector General declined CNBC's request for comment, stating its practice of neither confirming nor denying the existence of investigations.

A person familiar with the matter told the newspaper that Justice Department Inspector General Michael Horowitz is now spearheading an investigation into the classification of the documents.

Comey has said he considers the documents to be personal documents. But in a Jan. 7, 2017 email containing the first memo he wrote, Comey said he was "not sure of the proper classification here so have chosen SECRET."

It is not yet clear when the Justice Department determined that Comey's memos contained classified information. The intelligence community often returns to documents and reclassifies them after further review.

Comey sent a total of four memos to his friend Daniel Richman, a former federal prosecutor, people familiar with the matter told the Journal.

Read the full report from the Wall Street Journal.