OAKLAND (CBS SF) — An investigation into alleged sexual misconduct by Oakland police officers has been widened to include a fourth officer who has been placed on leave, the department said Monday.

Oakland police Chief Sean Whent said on Friday that three officers were placed on paid administrative leave in the past month while the department investigates allegations that they were involved in sexual misconduct with a woman who may have been underage during the alleged encounters.

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Police haven’t released the names of those three officers or the fourth officer who is now also part of the probe.

Whent, who spoke at a news conference at City Hall with Oakland Mayor Libby Schaaf, said the allegations surfaced after police investigated the suicide by Officer Brendan O’Brien last Sept. 25.

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The department said in a statement on Friday that “evidence was recovered that led to the opening of an Internal Affairs investigation into allegations of potential sexual misconduct.”

The department said today that it is “committed to transparency: but added, “a complete investigative process requires confidentiality in order to maintain the integrity of the investigation.”

The department said, “For this reason, only those preliminary details that do not compromise the investigation can be released at this time.”

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Whent said police also investigated the apparent suicide of O’Brien’s wife, Irma Huerta-Lopez, at the couple’s apartment in the 8000 block of Greenridge Drive in the Oakland hills on June 16, 2014, to see if it could have been a homicide but senior investigators determined that it was a suicide.

When said there were two gunshot holes at the couple’s apartment but said it’s “not uncommon” for people who commit suicide to fire two shots.

He said gunshot residue was found on the hands of both Huerta-Lopez and O’Brien, but he said crime lab technicians had expected to find residue on O’Brien’s hands as a result of his routine handling of firearms as a normal part of his duties as an officer.

Police also said that surveillance video and receipts corroborated O’Brien’s account that he as at a convenience store at the time of his wife’s death.

Schaaf said on Friday that the sexual misconduct allegations as well as several other incidents in which officers have allegedly engaged in misconduct off the job are “very disturbing” and could hurt the Police Department’s effort build trust with the public and complete the reforms it has been making to comply with the 2003 settlement of a major misconduct lawsuit.

Schaaf said that as of Friday, the Alameda County District Attorney’s Office agreed to conduct parallel and independent investigations of the sexual misconduct allegations as well as of the apparent suicides of O’Brien and Huerta-Lopez.

She also said that to make sure that the district attorney’s investigations are unbiased, no former Oakland police officers who work as investigators for the district attorney will be involved in the probes.

In addition, Schaaf said she issued an executive order requiring that the district attorney’s office be notified of any future suspicion of wrongdoing on the part of Oakland officers.

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