MEXICO CITY — As schools resumed classes last week , teachers, students and parents arrived at Rafael Veloz Elementary in Ciudad Juárez wearing white, holding hands and weeping. The school’s head teacher, Elsa Mendoza Márquez , had been killed on Aug. 3 in the mass shooting in El Paso. The parents had to explain to their young children why she was missing. The assistant head told them that Ms. Mendoza would always be in their hearts.

Ms. Mendoza, 57, was one of eight Mexican citizens among the 22 people who died in the attack, in which a man named Patrick Crusius is accused of shooting shoppers in a Walmart with an AK-style rifle. Most of the other victims were Americans of Mexican descent, while one was a German. Ms. Mendoza had crossed the border to see family in El Paso, according to reports, and popped into the store while her husband and son waited in the car.

The El Paso shooting was at the top of the agenda of the first meeting between President Andrés Manuel López Obrador and the new American ambassador, Christopher Landau, on Monday. (The last ambassador, Roberta Jacobson, resigned in May 2018, and has since been a vocal critic of President Trump.)

Mexico’s position is that justice for the El Paso victims must include a condemnation of the hate that the authorities say drove Mr. Crusius to mass murder. “We think this case is decisive so these acts are not repeated,” said Mexico’s foreign secretary, Marcelo Ebrard.