A peaceful march at the border of Mexico and the United States on Sunday devolved into chaotic scenes of American border protection agents firing tear gas into a crowd that included children. Rights experts and others have questioned whether this use of force was justifiable or legal.

The episode, at the San Ysidro crossing that links San Diego and Tijuana, Mexico, became a focus of outrage, particularly after a photograph of a woman and her young daughters being tear-gassed attracted international attention. One of the children was barefoot and both were still in diapers.

President Trump and Kirstjen Nielsen, the secretary of Homeland Security, have defended the firing of tear gas. But others like Gavin Newsom, the governor-elect of California, have denounced the action. The Foreign Ministry of Mexico has asked the United States for a full investigation.

Is it legal to fire tear gas across an international border?

Dozens of tear gas canisters fired by the Americans were visible on the Mexican side of the border immediately after the episode. Legal experts have questioned the justification for using such weapons against people who were not in the United States.