In late 2023, Dena Hernandez returned to her room inside the women’s prison in Chowchilla, Calif., and spotted a packet in the mail. It contained a letter saying the Los Angeles district attorney was looking for incarcerated women who might be eligible for shortened sentences. “Your case,” the letter read, “has been identified for initial review.”

Ms. Hernandez was in shock. She was 13 years into a 28-year prison sentence for carjacking. Hours away from her family, she had received just one visit. There were no lawyers puzzling over her case or community groups pleading for clemency.

“Is this real?” Ms. Hernandez asked her roommate.

The letter was from an organization called For the People. It said there was a law in California allowing prosecutors to revisit old sentences that are “no longer in the interest of justice.” After going into effect in California in 2019, the law passed in four more states, part of a wave of criminal justice reform efforts washing over the United States.

But of all of the people who had come home under these resentencing laws, very few were women. Ms. Hernandez’s case was about to become part of an effort in California to change that.

Ms. Hernandez, 33, is stocky with square shoulders and a geometric tattoo etched into the side of her neck. Just above her right wrist is the shadow of another tattoo she removed. It once read “El Monte,” the name of the Southern California town where she grew up and the street gang that defined her childhood.