Mexico has begun taking steps to increase security near its border with the United States. Mexican officials announced Sunday that police arrested 39 people after a peaceful march turned violent. The march was called to protest long processing delays for immigrants seeking asylum in the United States. The violence began when a group of migrants broke away from the protesters and attempted to cross the border. U.S. border agents reacted by firing tear gas into Mexico. U.S. Homeland Security Secretary Kirstjen Nielsen said Customs and Border Patrol agents were struck by objects thrown by members of the group. The agency said that officers decided to use the tear gas “because of the risk to agents’ safety.” Mexico’s Interior Ministry said that officials were able to contain a group of about 500 people who “violently” tried to cross the border. The ministry said that those found to have taken part in the violence would be expelled immediately.