Federal investigators have executed search warrants at Baltimore Mayor Catherine Pugh's residences and Baltimore City Hall. The FBI has confirmed it is working with the IRS. Sources confirm the search warrants are part of a criminal investigation into the mayor.

Pugh has come under scrutiny in recent weeks for book deals that paid her hundreds of thousands of dollars. She received payments from several companies and entities for her "Healthy Holly" children's books totaling at least $700,000.

Federal law enforcement sources confirmed to CBS News that the book deal is part of the investigation but that there is "more to the picture." It is unclear what other aspects of her actions are now under scrutiny. The FBI has not confirmed details contained in sealed search warrants.

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Pugh, who has been on a leave of absence since April 1, has refused to step down amid calls for her resignation. CBS Baltimore reported that Baltimore City councilmembers signed a letter earlier this month calling for Pugh to step down.

Baltimore Mayor Catherine Pugh seen Aug. 16, 2017, in Baltimore, Maryland. Getty

"The entire membership of the Baltimore City Council believes that it is not in the best interest of the City of Baltimore, for you to continue to serve as Mayor," the letter stated. "We urge you to tender your resignation effective immediately."

The FBI's probe comes amid several investigations into Pugh's business dealings, including probes by the state prosecutor's office, the City's Board of Ethics, the City's law department, and the state insurance commissioner, as well as an internal audit from the University of Maryland Medical System.

Despite the inquiries, Pugh released a statement saying she "fully intends to resume the duties of her office."

"Mayor Pugh has taken a leave to focus on recovering from pneumonia and regaining her health. She fully intends to resume the duties of her office and continuing her work on behalf of the people and the City of Baltimore," Pugh's spokesman told CBS Baltimore earlier this month.