2015/5/8-First came disclosures of racist and homophobic text messages exchanged by officers of the San Francisco Police Department. That was followed by the discovery that sheriff’s deputies had been gambling on forced fighting matches between inmates at a city jail.

Then on Thursday, the San Francisco district attorney announced that he was expanding the investigation of the city’s police and sheriff’s departments to examine whether those agencies have a deep-seated culture of systemic bias that has led to unlawful arrests or prosecutions.

In a year in which many of the nation’s major cities have been rocked by protests after the fatal police shootings of unarmed African-Americans, the broadened inquiry made clear that even a city known for its liberal politics can be buffeted by accusations that its officers behaved in a racially biased manner.

African-Americans in San Francisco have complained for years about harassment and the use of excessive force by the police. And while African-Americans make up about 5 percent of the city’s population, they account for half of its arrests and jail inmates, and more than 60 percent of the children in juvenile detention, according to city statistics.

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In Baltimore on Wednesday, Mayor Stephanie Rawlings-Blake acknowledged a “fractured relationship between the police and the community” in her predominantly black city and asked the Justice Department to conduct a civil rights investigation of the Police Department to determine whether officers had engaged in unconstitutional patterns of abuse or discrimination.

At a news conference in San Francisco announcing the expanded inquiry, the district attorney, George Gascón, acknowledged that the racist text messages had particularly undermined public confidence in both his office and the local criminal justice system.

He also said he believed that the city’s tradition of inclusiveness would allow it to avoid the tumult in Ferguson, Mo., and other cities where racial bias has been found to have played a role in the actions of police officers.

“In the last few months, we have seen city after city where police use of force or other police activity is coming to the light and indicating that racial animosity and other types of biases play a significant role,” he said. “I think at one point we felt we would be immune from that type of activity.”

Concerns that the San Francisco Police Department may be rife with racial bias were reignited in March when racially inflammatory text messagessent between 14 police officers became public as part of the federal corruption trial of two San Francisco officers.

Mr. Gascón, a former San Francisco police chief, said Thursday that a task force of prosecutors had already been scrutinizing some 3,000 cases — including about 1,600 convictions — related to contacts or arrests made by the 14 police officers during the last decade to determine if biases had led to any unlawful arrests or wrongful prosecutions.

The investigation by the panel, which will add three former judges as investigators, will now be broadened to include an examination of whether entrenched biases exist in the 2,000-member department.

“If just one individual was wrongly imprisoned because of bias on the part of these officers, that’s one too many,” Mr. Gascón said. “What is the potential impact in our justice system when a juror in a criminal trial questions the credibility of the arresting officer on the evidence that is being presented because they believe that this process may have been influenced by racial or homophobic bias? Can justice prevail under such conditions? Probably not.”

The text messages the officers exchanged discussed lynching African-Americans and proposing that African-Americans “should be spayed.” One text read “White Power.” Some referred to African-Americans using a racial slur.

Other texts contained denigrating comments about gays, Mexicans and Filipinos, who make up a significant number of residents in one of the nation’s most culturally diverse cities.

In recent months, there have also been a number of cellphone videos posted online of San Francisco police officers apparently mistreating citizens — many of them African-American — including one in which an officer nearly tipped a disabled man out of his wheelchair onto the street.

“Shame, shame, shame on San Francisco,” Rev. Amos C. Brown, president of the San Francisco office of the N.A.A.C.P., said at the news conference. “We cannot claim with integrity and honesty that we are first-class, inclusive, loving.”

Greg Suhr, the police chief, had no immediate comment Thursday, but hehas moved to fire seven officers who sent and received the racist text messages. An eighth officer has resigned.

“We have cooperated with the district attorney and handed them the requested documents so they could conduct their audit,” the police department said in a statement. “The D.A. has to review the cases and it’s their responsibility to determine if there is any bias in those cases.”

In addition to the text messages, the task force is also investigatinggladiator-style fights among San Francisco jail inmates that the city’s public defender, Jeff Adachi, has said were arranged by sheriff’s deputies. The jail guards, according to a report by Mr. Adachi, bet on the fights and threatened inmates with violence or withheld food if they did not take part.

A third area being examined is the possibility that hundreds of convictions in criminal cases may have been compromised by analysts at the police laboratory who appear to have improperly handled DNA samples.

The broadening of the panel’s focus was met with relief by residents who have long questioned police behavior and arrests made in Bayview-Hunters Point and other African-American neighborhoods.

“Fighting for civil rights is really part of the San Francisco culture and legacy and so it only makes sense that we move forward on this,” said Malia Cohen, a member of the city’s Board of Supervisors who has urged a similar inquiry in the past.

The text messages were disclosed in March as part of a federal corruption case against Ian Furminger, the former sergeant who sent many of the messages.

Mr. Furminger, a 20-year veteran convicted in December 2014 of stealing money and property from suspects, has been sentenced to 41 months in prison. As part of the case, prosecutors revealed that he had sent and received a number of the text messages.

The expanded district attorney’s task force will now include Cruz Reynoso, a former California Supreme Court justice; Dickran Tevrizian, a retired federal court judge; and LaDoris Cordell, a former Superior Court judge, who was once a vice provost at Stanford University. The panel is expected to conclude the investigation by the end of the year, officials said.