The Baltimore City Board of Ethics has reportedly opened an investigation into Mayor Catherine Pugh, who recently announced an indefinite leave of absence.

The mayor's office on Monday announced she was leaving to focus on her health after suffering from pneumonia. That followed claims that she received $500,000 from the University of Maryland Medical System (UMMS) in what critics have called a "self-dealing" book sale.

BALTIMORE MAYOR CATHERINE PUGH TAKES INDEFINITE LEAVE OF ABSENCE AMID BOOK CONTROVERSY

The city's ethics board confirmed to Fox affiliate WBFF on Wednesday their decision to investigate whether Pugh, a Democrat, violated ethical codes over the deal.

Earlier this week, Maryland Gov. Larry Hogan, a Republican, called on the state prosecutor to investigate allegations of self-dealing by Pugh. State Comptroller Peter Franchot, a Democrat, urged the mayor to step down immediately.

Since 2011, Pugh has received $500,000 for selling the books to UMMS, The Associated Press reported. The $4 billion hospital network, one of the largest private employers in the state, reportedly paid Pugh for 100,000 copies of her books between 2011 and 2018.

The firm Pugh identified as her books' printer said it produced only 60,000 copies and did not have any more orders from Pugh. She had acknowledged sales of her book to UMMS and said they were meant to be distributed to city schools and daycares.

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Pugh, at a news conference last week, described the book deal with the university-based health care system as a "regrettable mistake," and apologized for "any lack of confidence or disappointment" citizens and colleagues may have felt.

Baltimore's acting mayor, City Council President Jack Young, said on Tuesday the city "is in good hands" until Pugh decides whether to return to office. He emphasized that he's only acting as a "placeholder" for the embattled mayor and plans to run for City Council president again.

The Associated Press contributed to this report.