Sheriff Lee Baca stunned the public this week by announcing that he will immediately retire as the head of Los Angeles County jails. His announcement marks another milestone in the ACLU's campaign to end the culture of rampant deputy-on-prisoner violence that has plagued the Los Angeles County jails for years.

L.A. County Jails are the biggest jail system in the nation – and maybe on the planet. It is surely one of the most notorious. This is a jail where deputies organized in gangs inside the jail savagely beat not only the prisoners but sometimes even prisoners' family members and others who come to the jail to visit their loved ones.

Jail observers were taken off guard by Baca's announcement: Despite repeated public exposures of terrifying abuses of power in the jails, the Sheriff has handily won re-election three times since 1998. He seemed so permanently entrenched that in the last election in 2010 no one even bothered to challenge him.

A couple of years ago Sheriff Baca telephoned me out of the blue, said that he happened to be in DC for a meeting down the street from my office, and could he stop by to say hello? It seemed an odd occasion for a social call since I was helping to lead the team of lawyers in the federal class action lawsuit against him. But he did stop by my office, we exchanged pleasantries about the weather, and as he was leaving he said with great earnestness: "Margaret, believe me: I will never, ever resign. I intend to be Sheriff as long as I live."

Finally, it looks like he's changed his mind. The perfect storm that led to Sheriff Baca's decision to resign has been brewing since 2008. That's when the ACLU began filing a series of public reports cataloging scores of beatings that a clique of deputies were administering to prisoners: shattering legs, fracturing skulls, gouging eyeballs, rupturing eardrums. In response to our reports, high-level jail commanders simply issued broadside indignant denials: the prisoners were all liars.

But in February 2011, an event occurred that would change everything. An ACLU paralegal actually witnessed two deputies viciously beat and Taser to unconsciousness a helpless prisoner – while they monotonously repeated "stop resisting" and "stop fighting" as though they were reading from a script. We reported this incident to the US Attorney's office, the FBI and the federal court. It was the first time that a highly credible civilian eyewitness had come forward, and media reports of the incident opened a floodgate of reports from other civilian eyewitnesses – clergy, teachers, and others who volunteer services in the jail, and had been too intimidated to speak out earlier.

In September 2011, the ACLU issued a massive report, titled "Cruel and Usual Punishment: How a Savage Gang of Deputies Controls LA County Jails." And in January 2012, the ACLU filed suit against Sheriff Baca , accusing the Sheriff of failing to stop the pattern and practice of deputy-on-prisoner violence.

The ACLU report and lawsuit resulted in a firestorm of media coverage and public outrage. In response, the Los Angeles County Board of Supervisors created the Citizens' Commission on Jail Violence, a blue-ribbon panel of former federal judges and prosecutors, tasked with gathering evidence, holding public hearings, and issuing findings and recommendations. In September 2012, the Commission issued a blistering final report, explicitly crediting the ACLU's allegations, and finding that "The problem of excessive force in the county jails lies with the department's leadership." In December 2013, federal prosecutors filed criminal charges against 18 current and former sheriff's deputies accused of beating jail prisoners and visitors and related crimes.

The Sheriff's office is now in negotiations with the ACLU to settle the ACLU's suit for injunctive relief with a permanent federally-enforceable decree. What was almost unimaginable in the recent past—far-reaching reform of the culture in the LA County Jails—is well on its way to becoming a reality. The Sheriff's wise decision to retire will help speed that day.