KIPP, one of the country’s largest and most successful charter school chains, dismissed its co-founder on Thursday after an investigation found credible a claim that he had sexually abused a student some two decades ago, according to a letter sent to the school community.

The co-founder, Michael Feinberg, was accused last spring of sexually abusing a minor female student in Houston in the late 1990s, according to someone with close knowledge of the case who was not authorized to speak publicly and asked not to be identified. An outside investigation found her claim credible after interviewing the student and her mother, who both gave the same sequence of events.

Mr. Feinberg denies the accusation, his lawyer, Christopher L. Tritico, said.

Investigators also uncovered evidence that Mr. Feinberg had sexually harassed two KIPP employees. One case, in 2004, led to a financial settlement, the letter said; the other could not be corroborated because the woman involved would not cooperate, but the letter found it to be credible.

“We believe that Mr. Feinberg’s actions were incompatible with the leadership qualities that are central to our mission,” said the letter, which was sent Thursday afternoon to teachers, administrators and families of students.