Chicago Mayor Rahm Emanuel spoke to the press while campaigning in February. He said he was not personally involved in the controversy that led the CEO of Chicago Public Schools to resign. Reuters

The CEO of the Chicago Public Schools, Barbara Byrd-Bennett, has resigned, effective immediately, amid a federal probe into a $20.5 million no-bid contract awarded to her former employer. Byrd-Bennett, who has been on paid leave since mid-April, delivered her resignation notice Friday. Her leave was scheduled to end next week, though she was already not expected to return to her post.

The contract in question was awarded to a nonprofit educational organization, SUPES, to train CPS principals through 2016. Byrd-Bennett hasn't been charged with any criminal wrong doing so far. Chicago Mayor Rahm Emanuel said Monday in a press release following the resignation that he was "saddened by the circumstances that have led to Barbara’s resignation and I wish her well."

Gary Solomon, the owner of the contracted company, SUPES, was instrumental in the process that brought Byrd-Bennett into her post as CEO in 2012, according to reports. Byrd-Bennett and Solomon were very close professionally, the Chicago Sun Times reported. Byrd-Bennett joined CPS after working at two of Solomon's companies.

A statement released by the Chicago Teachers' Union called Byrd-Bennett's behavior leading up to the controversy a poor example for the students in the CPS system as well as for administrators who looked to her for leadership. The union also said that it was "not surprised" by her resignation. While CPS looks for a replacement, interim CEO Jesse Ruiz will continue in her stead.

Documents related to some of Byrd-Bennett's top deputies are being sought as part of the federal probe, according to the Chicago Tribune.

SUPES specializes in leadership training programs and is well known for its role in what supporters call school reform efforts. Emails obtained by the Chicago Tribune showed SUPES played up its clout in the Emanuel administration and with Byrd-Bennett before it received the noncompetitive contract just weeks later. Emanuel has denied any connection himself with the contract negotiations in question.