EL FUERTE, Mexico—One recent day, a line of grieving mothers armed with picks and shovels worked their way across a muddy field looking for Mexico’s dead and missing, their own children among them.

“It smells bad here,” said Lizbeth Ortega, a member of Las Rastreadoras de El Fuerte, or the Trackers of El Fuerte, a group of mothers who look for missing people.

The...