OAKLAND, Calif. (KRON) — Oakland’s new mayor, Sheng Thao, announced at a press conference Wednesday that she fired the city’s police chief, LeRonne Armstrong.

Thao said she lost confidence in Armstrong because of how he failed to discipline a sergeant who was involved in a hit-and-run crash, and for how the chief later downplayed the seriousness of the sergeant’s alleged misconduct.

The mayor announced she decided to “separate Chief Armstrong from the city without cause. This was not an easy decision.”

A federal oversight monitor’s investigation concluded that the Oakland Police Department “repeatedly failed to rigorously investigate misconduct and hold officers accountable.” OPD has “serious flaws in the disciplinary process,” investigators concluded.

Thao said Armstrong’s reaction to the independent investigation was troubling. “Chief Armstrong made a number of statements that troubled me. Chief Armstrong said these were not incidents where officers behaved poorly. He stated that he did not believe these incidents reflected systemic problems,” Thao said.

“It is clear to me that there are systemic issues. We cannot simply write them off as mistakes,” Thao said.

The mayor said, “I made a commitment as your mayor to ensure the police department and city can prove, once and for all, that Oakland can ensure constitutional policing without federal oversight. Our police department has made great strides. But there is so much more to be done, and to do.”

In January, Thao placed Armstrong on administrative leave after an investigation by Federal Monitor Robert Warshaw concluded that the police chief failed to handle misconduct allegedly committed by the sergeant.

Armstrong held his own press conference with reporters last month and declared he did nothing wrong. “I should be reinstated immediately in the interests of justice,” he said.

The Oakland Police Department has been subject to court-appointed federal oversight for the past two decades. Armstrong accused Warshaw of creating a fake scandal to extend federal oversight over the OPD.

“This is not a scandal. This to me, clearly, is a last-ditch effort to destroy the credibility of me, destroy the credibility of this department, and to make the community believe that, again, OPD is involved in some shady business,” Armstrong told reporters in January.

But the mayor told reporters on Wednesday, “I am no longer confident that Chief Armstrong can do the work needed to achieve the vision.”

Thao said she still respects Armstrong’s service “to the city that he grew up in and that he clearly loves dearly. It’s precisely because I admire Chief Armstrong that this has been personally very difficult.”

An hour after the mayor’s announcement, Armstrong sent KRON4 a prepared statement through his attorney. Armstrong wrote, “I am deeply disappointed in the Mayor’s decision. After the relevant facts are fully evaluated by weighing evidence instead of pulling soundbites from strategically leaked, inaccurate reports, it will be clear I was a loyal and effective reformer of the Oakland Police Department.  It will be equally clear that I committed no misconduct, and my termination is fundamentally wrong, unjustified, and unfair.”

Armstrong said he will release more details once he has enough time to reflect on the mayor’s remarks.

Armstrong has vocal critics and supporters.

The NAACP Oakland Branch held several rallies in support of Armstrong and sent letters to Thao calling for his reinstatement. Armstrong’s leadership was critical for the OPD’s stability and focus on reducing crime, NAACP Oakland Branch President Cynthia Adams said.

In response to the chief’s firing, Adams wrote, “We believe that this action moves Oakland in the wrong direction by eroding community trust, destabilizing public safety by destabilizing OPD leadership, and undermining the power and independence of the Oakland police commission.”

Cat Brooks, Executive Director and co-founder of the Anti Police-Terror Project, applauded Thao’s decision as a “bold and courageous move.”

“Despite attempts by Armstrong, who was more busy being a political actor than a police chief, to coerce the Mayor into preserving his job, Mayor Thao remained rooted in the facts of the investigation. This is an important first step in decades of little-to-zero accountability for this Department,” Brooks said.

The mayor and police officers’ union said Darren Allison will continue to lead the OPD as acting chief of police.

Oakland Police Officers’ Association President Barry Donelan said, “Despite the turmoil surrounding the leadership of the police department, Oakland residents can be assured that Oakland’s hard working police officers have remained on duty throughout; responding to 911 calls, investigating crime, and will continue with their dedicated service to our city.”