The FBI led a series of raids Thursday targeting police officers in Northern California following an investigation into alleged civil rights violations by law enforcement officers. 

Police officers from Antioch and Pittsburg were arrested after a federal grand jury in San Francisco handed down an indictment that accuses current and former officers with a wide range of offenses, including criminal conspiracy, The Mercury reported. 

In a statement, Antioch Mayor Lamar Thorpe called Thursday "a dark day in our city’s history, as people trusted to uphold the law, allegedly breached that trust and were arrested by the FBI."

Thorpe has been outspoken on his desire to reform his city's police department. 

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Antioch police vehicle

An Antioch police vehicle. Officers with the police department and Pittsburg Police Department, both in California, were arrested Thursday following an FBI raid.  (Antioch Police Department)

"To those that have accused me and others of being anti-police for seeking to reform the Antioch Police Department, today’s arrests are demonstrative of the issues that have plagued the Antioch Police Department for decades," he said. "Seeking to reform the Antioch Police Department is not anti-police, it is pro our residents, and pro officers that have served and continue to serve with honor."

Fox News Digital has reached out to the FBI, and the Pittsburg police chief and city manager, as well as other local officials. 

The arrests followed an 18-month investigation that began when a tipster when a tipster informed the FBI and Contra Costa District Attorney that a group of East Contra Costa County police officers were cheating on college tests to obtain education incentive pay bumps, the news report said.

Upon seizing cellphones from officers, investigators allegedly found racist and homophobic text messages. As many as 45 of Antioch's approximately 100 officers were placed on leave because of the texts, Fox San Francisco reported. 

Among the allegations are text messages sent between Antioch officers, using racial slurs and describing violence against suspects, and going as far as threatening Thorpe, who is African American, the report said. The N-word was used at least a dozen times, as were terms describing African Americans as "gorillas."

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The probe later widened to include alleged drug trafficking and violent crimes.

As a result of alleged police misconduct, federal and state prosecutors have dropped or dismissed dozens of cases that relied on the impugned officers. 

Antioch is already the subject of a federal civil rights lawsuit requesting federal oversight of its police department. In May, California Attorney General Rob Bonta announced a civil rights investigation into the city's police department to determine if it has "engaged in a pattern or practice of unconstitutional policing amid deeply concerning allegations relating to bigoted text messages and other potentially discriminatory misconduct."

"Police departments are on the front lines of that fight every day as they work to safeguard the people of our state," Bonta said at the time. "However, where there are allegations of potentially pervasive bias or discrimination, it can undermine the trust that is critical for public safety and our justice system. It is our responsibility to ensure that we establish a culture of accountability, professionalism, and zero tolerance for hateful or racist behavior, on or off duty."

John Burris, a civil rights attorney who has filed a civil rights lawsuit against the Antioch Police Department, said the arrests make the case for the need for federal oversight of the city's law enforcement agency. 

"This conduct is representative of the mindset that these officers had," he told Fox News Digital. "Those racial text messages were pretty horrendous and really went to the lack of respect that they had for the community they served. What you then have is a dishornable cop who's also a racist."

Shagoofa Khan, a 23-year-old community activist who grew up in Antioch, told the newspaper that a tracker was put on her car and disparaged her in racist and sexualized text messages. She said she was relieved upon hearing news of the arrests. 

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"Finally," she said of the indictments. "This process needs to conclude before the community can heal."