Here in Nuevo León, one of the states hardest hit by violence, they have praised the prosecutor’s office for working with a local human rights group and family members to review several dozen cases, sometimes performing investigations that were never done in the first place.

Another look at Ms. González’s case last year helped lead the police to a gang leader who was arrested in January. He confessed to abducting and killing the women, and directed the authorities to the remains of the cousin; further testing of bones at the site is under way to determine if any belong to Ms. González’s daughter. “Whatever God may want,” Ms. González said through tears, “whether my daughter is alive or dead, I am resigned to whatever may come.”

Since agreeing in June 2011 to reopen dozens of cases, out of thousands here in the past several years, the authorities and their civilian counterparts have met monthly to go over leads and any progress made on them. Fifty-two cases have been resolved, with some people found dead and others alive, including 12 this year who were discovered to be in the custody of the authorities, unknown to their families. About 40 people have been arrested on abduction or homicide charges, 16 of them police officers.

“Nuevo León is one of the only states where you see prosecutors actually doing the due diligence of conducting investigations, meeting with families, going to the crime scene, taking common-sense steps to advance the investigation,” said Nik Steinberg, an investigator with Human Rights Watch, which in February published a damning report on the disappeared. “To search for the missing and find the people responsible for taking them, in Mexico where normally investigators don’t do any of that, that is progress.”

Mexico has only a rough sense of how many people have disappeared amid a surge of violence over the past several years that has left tens of thousands dead in battles between drug gangs, organized-crime groups, the police and the military.