Mike Feinberg, the co-founder of the KIPP charter network, has been fired after an investigation into sexual misconduct, its leaders announced Thursday.

KIPP found “credible evidence” connected to allegations that Feinberg abused a student in the late 1990s, according to a letter sent to students and staff. Feinberg denies the allegations.

“We recognize this news will come as a shock to many in the KIPP Team and Family as we struggle to reconcile Mr. Feinberg’s 24 years of significant contributions with the findings of this investigation,” the letter says.

It’s a stunning move at one of the country’s best-known charter school organizations — and one where Feinberg has been in a leadership role for more than two decades. Feinberg started KIPP along with Dave Levin in Houston in 1994, and Levin brought the model to New York City the next year. The network became known for its “no excuses” model of strict discipline and attention to academic performance.

KIPP says it first heard the allegation last spring. The network eventually hired the law firm WilmerHale to conduct an external investigation, which found evidence that Feinberg had sexually harassed two adults, both alums of the school who were then employed by KIPP in Houston, the network said.

“In light of the nature of the allegations and the passage of time, critical facts about these events may never be conclusively determined. What is clear, however, is that, at a minimum, Mr. Feinberg put himself into situations where his conduct could be seriously misconstrued,” KIPP wrote in the letter, signed by CEO Richard Barth and KIPP’s Houston leader, Sehba Ali.

Feinberg’s lawyer, Chris Tritico, told the Houston Chronicle that Feinberg had not been fully informed about the allegations against him.

“The treatment he received today from the board that he put in place is wrong, and it’s not what someone who has made the contributions he’s made deserves,” Tritico said.

Read KIPP’s full letter here.