The deputies union and Supervisor Ray Mueller confirmed Fox's resignation.
The Sheriff's office tells ABC7 News it "does not comment on personnel matter," and that Sheriff Corpus would "like to thank him for his dedicated service to the San Mateo County community and she wishes him the best in his future endeavors."
TIMELINE: San Mateo County Sheriff Christina Corpus investigation
"We are actually very concerned about morale in the sheriff's department with this amount of turnover taking place in the command staff," Mueller said.
Fox was promoted to his role of assistant sheriff in September, the deputies union said. According to his biography page on the department's website, he joined the office in 2023 and has more than 25 years of law enforcement experience in the area.
ABC7 News reached out to Fox for comment about his departure and will update this story if we hear back.
Additionally, Mueller confirmed to ABC7 News that an unnamed officer also left the sheriff's office, after he was asked to arrest Deputy Carlos Tapia. On Tuesday, the sheriff's department arrested Carlos Tapia, who is also president of the Deputy Sheriff's Association union, on two separate charges.
Tapia is one of many deputies who has criticized the sheriff and her office.
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It all follows a week of tumult in the department after a nearly 400-page investigative report accused his boss, Corpus, of a pattern of misconduct. Some of the allegations include abuse of power, retaliation, having an inappropriate relationship with her chief-of-staff, and using racist and homophobic slurs. The report was prompted by internal complaints against Corpus and commissioned in July. The county tapped retired Santa Clara County Judge LaDoris Cordell to look into fifteen allegations from current and former employees in the sheriff's department.
Via phone call, Corpus told ABC7 News she feels she has been unfairly targeted in this investigation and denies all wrongdoing.
"Given the disarray in her executive management team, there are some severe problems of whether or not she can answer if she's fulfilling her duty to the residents of this county," Mueller said.
The calls for Corpus to resign were echoed by Heinz Puschendorf - a former employee and previous head of the sheriff's department union.
He says the department has been plagued by scandal for years.
"We were hoping that she was going to make a change. And in fact, she's just as bad as the past history has always been," Puschendorf said.
RELATED: San Mateo Co. sheriff won't resign, promotes controversial staffer to asst. sheriff amid scandal
Puschendorf says he still talks with many people who work at the sheriff's office.
He tells ABC7 News he believes the report is an accurate depiction of the department.
"With the way everything has been crumbling and falling around her, I would expect the best thing would be to step out of the way and hopefully they can get at least a couple more people in to help organize so they can restructure," said Puschendorf.
Corpus has remained defiant throughout the week, refusing to cede to mounting pressure for her to resign. During a Wednesday meeting in which the board of supervisors unanimously passed a vote of no confidence in her, Corpus stormed out of the meeting after brief remarks in which she called the report a "hatchet job" that is "filled with lies."
During the meeting, supervisors were also about to remove the position of Corpus' chief of staff, Victor Aenlle, before she announced she would promote him to assistant sheriff. After that, the board passed a resolution directing its legal counsel to look into whether Corpus legally has the ability to do that.
The board also referred the investigative report to the District Attorney's office, as well as the California Attorney General's office.
RELATED: San Mateo Co. officials call on Sheriff Christina Corpus to step down following explosive report
If Corpus does not resign, supervisors are planning to bring a charter amendment before the public. This would appear on the ballot in a special election as early as March and would ask voters to approve or deny the San Mateo County Board of Supervisors' ability to remove the sheriff under certain circumstances. If approved, the board would have this power in the future to remove another sheriff if they violate the specified parameters defined in the potential charter amendment.
"We are focused on trying to bring stability back to the situation as soon as possible," Mueller said.
Policing expert and San Jose State University professor Greg Woods says if she stays on, he wouldn't be surprised to see more deputies leave the department.
"Those individuals who put their professional reputations on the line are concluding that don't wish to any longer associate with that particular agency," Woods said.
While the scandal continues to unfold, many have raised questions about what it could mean for San Mateo County residents and the communities that rely on the sheriff's office.
In Half Moon Bay, City Manager Matthew Chidester says he doesn't anticipate it having an impact on public safety.
"The men and women of the sheriff's office will be committed to continue to provide services to the coast and keeping our residents safe," Chidester said.
While the county can't fire the sheriff, the Board of Supervisors is looking into ways to have her removed.
Read the full investigation report here.
Editorial Note: This article was updated to remove a report of Undersheriff Dan Perea resigning.
This is a developing story, check back for updates.