Last Monday, Brandon Giovanni decided to accompany his stepfather on the Metro to his evening shift in a restaurant downtown. For the seventh-grader, it was a break from the cramped family flat in Colonia Zapotitla, a sprawling neighborhood of unpaved roads and cinder block dwellings in this vast capital’s gritty southeast borough of Tláhuac.

Though he didn’t say so, Marisol Tapia knew that her son had another, secret motivation: He wanted to buy her a gift for Mother’s Day.

“Mamá, we’re headed to the Periférico station,” an animated Brandon Giovanni replied when his mother called on his cellphone about 10:15 p.m., as he and his stepfather were headed back home on the Metro Line 12. “I have a surprise for you!”

Periférico was only three stops from the Nopalera station, where the pair would exit.