This confusion tears at the students’ families; they have no closure. Across Latin America, mothers still search for their children who disappeared in “dirty wars” that governments waged in the 1970s and 1980s. It is chilling that cartel killers, and the corrupt police officers they work with, use this same tactic of disappearing victims today. The gangsters don’t just disappear people to hide evidence; it is also a potent form of terror.

Mexican agents have arrested more than 100 people suspected of having taken part in the Iguala attacks, including city police officers, the mayor and his wife, and reputed Guerreros Unidos assassins. Some have confessed. But accusations that they were tortured undermine the cases against them. If some are the killers but are freed because of a bungled prosecution, it would only add to the tragedy. The case needs to be strengthened with solid physical evidence. State and federal forces also need to be more thoroughly questioned about their actions on the night of the abductions.

The Iguala report questions the motives behind this brutal attack. The students, from a nearby teachers’ college, had gone to the town to commandeer buses that they wanted to use to travel to a march in Mexico City. The report recommends pursuing the theory that students unwittingly took a bus in which heroin, or its precursor, opium paste, was stashed. The Guerreros Unidos produce heroin to supply the surge in use across the United States.

Prosecutors have also cited an alleged Guerreros Unidos hit man, who said the group (mistakenly) believed the students had been infiltrated by a rival gang. Yet another theory is that the police were trying to stop students from disrupting a public event held by the mayor and his wife, both of whom are believed to have been in league with the cartel.

Whatever drove the killers that night, cartels and corrupt police officers have a track record of disposing of innocent civilians, sometimes for just being in the wrong place. After his son was killed by cartel thugs, the poet Javier Sicilia toured cities across Mexico packed with thousands grieving over their loved ones. According to a government count, cartels and the security forces fighting them killed more than 83,000 people between 2007 and 2014. Some journalists claim it is many more.

The failures in the Iguala investigation have taken a toll on Mr. Peña Nieto, who has fallen to a 35 percent approval rating. He did not invent the problem of cartel bloodshed when he took power in 2012. But, as president, he needs to take responsibility and lead real efforts to fix the justice system during his remaining three years in power.

And when the United States supports Mexico in fighting cartels, it cannot just provide hardware like Black Hawk helicopters. It needs to help Mexico rebuild its institutions from the bottom up.