But his efforts had caught the attention of San Diego’s city attorney at the time, John Witt. Mr. Witt called Mr. Gwinn to his office and told him it was going to be difficult, but he understood what Mr. Gwinn had tried to do. “He told me to go out and figure out how to win these cases,” Mr. Gwinn said.

Mr. Gwinn began ordering 911 tapes in all domestic violence cases. He asked the police to take pictures of everything: the crime scene, the victims, even the perpetrators raging in the backs of police vehicles. Any possible shred of evidence that existed, Mr. Gwinn wanted. He began to go out to local police departments to enlist them in his mission. When one sergeant told Mr. Gwinn that he was never going to prosecute these cases successfully, Mr. Gwinn created a messaging system to let the police know how their cases were resolved. It gave the officers a sense of agency, learning their efforts could actually make a difference.

Mr. Gwinn tried 21 cases in a row, all domestic violence misdemeanors. All without the victim testifying.

He won 17 of them.

By the mid-1990s, Mr. Gwinn had become a national leader in evidence-based prosecution; he and a colleague trained thousands of lawyers around the country. He believed fervently that if we could prosecute murderers without a victim’s cooperation, we could prosecute batterers.

The movement gained momentum across the country, particularly in left-leaning states and states with stricter domestic violence laws. Still, there were many rural and conservative areas where it hadn’t gained much traction — places like Montana.

Could evidence-based prosecution have saved Michelle Monson Mosure and her children? In addition to her affidavit, had anyone investigated further after she recanted, they might have learned how Rocky had threatened his family once with Michelle’s grandfather’s gun, or how he’d sometimes take the children as leverage to coerce Michelle into obeying him. They might have learned he was stalking his wife when she left the house, isolating her from friends and family — all of which could have come together to paint a picture of a family in serious danger.

In the end, it’s impossible to say whether Michelle could have been saved. But three years after her death came Crawford v. Washington, the case that nearly crushed two decades of progress.