OAKLAND — A sexual misconduct investigation has ensnared a fourth Oakland police officer, the department said Monday.

The Oakland Police Department did not name the officer or the three others who were put on leave over the past weeks. The allegations of sexual misconduct involving the officers surfaced after the September suicide of Officer Brendan O’Brien.

“The Oakland Police Department’s Internal Affairs Division has placed a fourth Oakland police officer on administrative leave as a result of our ongoing investigation into a recent misconduct case,” the department said in a statement Monday. “The Oakland Police Department is committed to transparency. However, a complete investigative process requires confidentiality in order to maintain the integrity of the investigation. For this reason, only those preliminary details that do not compromise the investigation can be released at this time.”

O’Brien, 30, left a note naming specific officers who allegedly were involved with the daughter of a police dispatcher who goes by the name Celeste Guap, according to a source who read the note.

Oakland police initially investigated but did not take any action. However, since then, the independent monitor overseeing department reforms learned of the allegations and raised questions.

Police have reopened the internal affairs and criminal investigations into the case, including trying to determine if Guap was a minor at the time. And on Friday, Mayor Libby Schaaf said the Alameda County District Attorney’s Office was going to conduct an independent review of the sex allegations, and deaths of O’Brien and his wife, Irma Huerta Lopez, who committed suicide a year earlier. O’Brien, who shot himself inside his Oakland apartment in September, lost his wife in June 2014 when she shot herself in the same place with his gun, according to autopsy reports.

Huerta Lopez’s death was briefly investigated as suspicious but later ruled a suicide. Her sister, Paulina Huerta, told this newspaper she still has doubts about Huerta Lopez’s suicide and has blamed O’Brien.

David DeBolt covers Oakland. Contact him at 510-208-6453. Follow him at Twitter.com/daviddebolt.