The gang truce in Querétaro was modeled after a similar pact among dozens of gangs in Monterrey. A nonprofit called Nacidos Para Triunfar played a crucial role in bringing gang members and civic authorities together.

The massive “Guadalupana,” as the annual celebration of the Day of the Virgin of Guadalupe is commonly known, would be a potential public health catastrophe.

Any source of criticism, whether a journalist, another politician or a member of civil society, can count on a barrage of invective from the president, senior members of his cabinet and often from among the millions of López Obrador’s online followers.

In Mexico, where both organized and petty crime has exploded to unprecedented levels, vigilante justice has become increasingly common; citizens who gun down assailants during robbery attempts often make headlines as heroes.

Mexico has long been plagued by often brutal violence against women and children. Just under 11 women are killed on average each day in Mexico because of gender-based violence.

At each of Mexico City’s 13 prisons, hundreds of people are still admitted each visiting day to see their imprisoned family members. For the inmates, they are a vital lifeline.

A shadow hangs over El Rosario where each year millions of monarch butterflies alight on the reserve's fir trees. Two local protectors of Mexico's monarch preserve have been killed, and so far, no one can say what happened to them.

Mr. LeBarón, the family's spokesperson, said he hopes he can channel the grief and anger over the killings into a broad social movement. “We want to unite the whole country. We want a social movement, not a political one,” he said.

Mexico is on edge after a wave of violence hit the country last week, culminating in heavy fighting between the army and alleged members of organized crime in Culiacán, the capital of the northern state of Sinaloa, that lasted for hours on Oct. 17.

The increased attention by Mexican police and armed forces is encouraging undocumented migrants to avoid shelters, many of which are run by the Catholic Church here, and to follow more dangerous routes through Mexico, aid workers warn.