Oakland’s acting police chief Paul Figueroa stepped down Friday, becoming the third head of the Oakland Police Department to abruptly leave the top post in nine days, just as another bombshell dropped: A new internal investigation is under way, this time into racist text messages and emails shared among officers.

A visibly frustrated Mayor Libby Schaaf revealed the probe at a news conference Friday night that she began with a simple declaration: “I am here to run a police department, not a frat house.” With that, she disclosed that her city’s department, already engulfed in a sex scandal, was also being probed for racist communications that were “wholly inappropriate and not acceptable for anyone who wears the badge of the Oakland Police Department.”

The texts were sent by African American officers, Schaaf said. She would not give other details, including whether members of the police command staff were implicated, saying state law governing public release of police disciplinary matters prevented her from doing so.

Some of the officers being investigated were “engaging in hate speech,” and others were “tolerating it” by receiving offensive messages and not reporting them, Schaaf said. One text obtained by NBC news showed an image of a Ku Klux Klan member on a cereal box with the message, “Brad, I heard you got boxes of these in your cupboards.” Another text appeared to show the word, “N—.”

In a move in which the mayor indicated she had lost faith in police leaders to run the department, she disclosed that she would not appoint another interim or acting chief to the top post. Instead, the department will have no chief, and for the time being command staff will report to City Administrator Sabrina Landreth as Oakland conducts a national search for a new chief.

“I feel that this is an appropriate time to place civilian oversight over this Police Department, to send a clear message about not tolerating misconduct, and to root out what is clearly a toxic, macho culture,” Schaaf said.

News of Figueroa’s decision came just hours after the explosive revelation that longtime Oakland police veteran Sgt. Mike Gantt had allowed his girlfriend to write some of his reports — and that she had shared confidential case information on social media, according to sources familiar with the matter. That development came amid an ever-expanding sexual misconduct case that has now implicated law enforcement officers at several other law enforcement agencies and at least one employee at a U.S. Department of Defense agency.

Schaaf declined to discuss Gantt’s case at the news conference, saying it would compromise her ability to seek the highest punishment possible. She said the department has referred cases he investigated that could be tainted to the Alameda County District Attorney’s office.

The mayor’s heightened emotion on Friday marked a stark contrast to the image of calm and control that she projected at City Hall last week after the sudden departure of Chief Sean Whent. Schaaf replaced Whent with interim Chief Ben Fairow, who she had not fully vetted and wound up firing after six days.

Several city officials were angry that the mayor had left them in the dark about Whent’s apparent ouster. Schaaf initially said the chief had resigned for personal reasons, but indicated Friday that she cannot fully disclose what happened because of a state law that guarantees confidentiality for police personnel files.

“I take ownership of my mistakes,” the mayor said Friday. “I hope I have been clear in what my intentions are.”

Meanwhile, the mayor has set her sights on significantly changing the department — ending what she says is a disgusting frat culture, even as the department has been under federal oversight for the last 13 years.

“We have a department with almost no women, no LGBT people, and a dramatic underrepresentation of people of color,” Councilwoman Rebecca Kaplan told The Chronicle. She is pushing for reforms that would require Oakland’s police force to rethink its hiring and recruiting strategies.

Indeed, many of the people caught up in recent disciplinary cases, including the sexual misconduct case involving a teenage sex worker, are rookie officers who were hired after 2013.

Over the past three years, Mayors Jean Quan and Libby Schaaf have made an aggressive push to boost the city’s police force, and Schaaf, in particular, established herself as a stalwart both for Whent and for the rank and file.

But with additional scandals bubbling up on what seems like a daily basis, she has struggled to restore public trust in the police force she worked so hard to build up.

“I think the community’s faith is completely destroyed in that department,” said Alameda County Public Defender Brendon Woods.

“This cannot go on,” said civil rights attorney Jim Chanin, who on Wednesday announced that he and fellow lawyer John Burris may ask a federal judge to intervene in hiring and recruitment for Oakland’s beleaguered department.

Yet others are confident that Schaaf will find a reformer from outside who can whip the force into shape.

“I think she can find someone who is really ruthless about instituting reforms of these kinds of things,” said Peter Keane, a former San Francisco police commissioner and law professor at Golden Gate University.

But that person probably has to come from another city, he cautioned. “You can’t do any of this stuff with an internal candidate.”

Chronicle staff writers Erin Allday and Demian Bulwa contributed to this report.

Rachel Swan, Phil Matier and Andrew Ross are San Francisco Chronicle staff writers. Email: rswan@sfchronicle.com, pmatier@sfchronicle.com and aross@sfchronicle.com