A vocal critic of Mayor Bill de Blasio and influential backer of New York City’s charter school sector has been fired from his job leading an education advocacy group after an investigation into “inappropriate behavior,” the organization said Wednesday.

Jeremiah Kittredge was ousted as CEO of Families for Excellent Schools after the organization received a complaint about “inappropriate behavior toward a non-employee” and retained outside counsel to investigate, board chairman Bryan Lawrence said in a statement.

“As a result of this investigation and additional factors, Jeremiah has been terminated as CEO,” Lawrence said.

Families for Excellent Schools officials did not answer questions Wednesday about the nature of the inappropriate behavior or what “additional factors” played a role in his dismissal. Kittredge did not return requests for comment.

The organization is well-funded, often considered a lobbying arm of the charter sector, and has deep roots in New York City. It has been closely aligned with Success Academy, the city’s largest charter network, and until recently had been an outspoken opponent of de Blasio and his approach to the charter sector.

Families for Excellent Schools also consistently criticized other major items on de Blasio’s education agenda. Kittredge has slammed the mayor’s support for “restorative” school discipline reforms over harsher steps like suspensions, arguing they have made schools less safe. And he has said the city’s high-profile school turnaround program “traps” students in failing schools.

The group’s advocacy efforts in New York City died down after the organization was hit with a record $426,500 fine in Massachusetts for failing to disclose its donors connected to a 2016 ballot measure to lift that state’s cap on charter schools. (Voters overwhelmingly rejected the ballot measure, a high-profile defeat for pro-charter groups.) It was not immediately clear whether that loss played a role in the decision to remove Kittredge.

Kittredge helped form Families for Excellent Schools in 2011, and the organization has received millions in funding from heavy hitters in the charter sector, including the Walton Foundation (Chalkbeat also receives funding from Walton).

The group has expanded to multiple states, including Massachusetts and Connecticut, and has served as a major organizing force among charter school parents. It’s staged rallies in Albany and launched an advertising campaign attacking de Blasio after he blocked a group of Success Academy schools from opening or expanding.