"The woman begged and pleaded with the National Guard to let them cross ... she wanted to cross to give a better future” to her young son Anthony Diaz, Gonzalez said. The soldier, dressed in desert fatigues, an assault rifle slung over his shoulder, said he was only following orders, according to Gonzalez.

The soldier did not disclose his name.

One of several images Reuters published of the scene, the photo was picked up widely on social media. It has thrown into the spotlight the role Mexico's militarized National Guard police force is playing in containing migration, mostly from Central America.

President Andres Manuel Lopez Obrador created the National Guard to bring down record homicide rates, but almost a third of its members are now assigned to patrolling the border to placate President Donald Trump's demands of stemming the flow of U.S.-bound migrants.