Why drug cartels want to slaughter students may at first seem inexplicable. But it is a symptom of a systematic process that has been taking place in Mexico for years. Drug cartels are taking over chunks of government apparatus, from local police forces to city and state governments. Sometimes, they control the officials; other times, cartel members themselves are the officials. I call it state capture. A student I talked to had a more visceral term for it: narco-politica, or narco-politics.

It’s a terrifying concept. Being ruled by corrupt and self-interested politicians can be bad. But imagine being ruled by sociopathic gangsters. They respond to rowdy students in the only way they understand: with extreme violence designed to cause terror. They stick the mutilated body of a student on public display in the same way they do rival traffickers.

The market city lies amid hills of marijuana and opium fields and is the fief of a brutal cell of traffickers who call themselves Guerreros Unidos, or Warriors United. After the discovery of the massacre of the students, federal soldiers took control of the city. Twenty-two police officers were detained for working with the cartel. In a brazen move, the Warriors put up banners calling for the release of the officers.

The Iguala police chief is now on the run with an arrest warrant behind him. The Iguala mayor has also fled town as the state moves to impeach him. An intelligence agency report linked him to the Warriors, the Mexican media revealed. His wife has also come into the spotlight. One of her brothers served prison time for trafficking and two others were killed in a gangland shooting, according to the intelligence report. Who knows how high this trail of corruption may lead?

The Iguala mayor was a member of the opposition Democratic Revolution Party. But the international attention to this atrocity is also an embarrassment for President Enrique Peña Nieto. Since taking power in 2012, he has been laboring to change Mexico’s violent image, focusing on reforms such as opening up the nation’s energy sector to foreign companies. He has also taken down major drug traffickers, such as Joaquín Guzmán Loera, known as El Chapo, and Nazario Moreno González, also known as El Más Loco. Some observers say Mr. Peña Nieto’s reforms have made this “Mexico’s moment.” But can it really be Mexico’s moment with such barbaric crimes against young people taking place? The president may be reforming Mexico’s laws, but this case highlights the deep problems in the institutions needed to uphold those laws.