The two men rang the bell at the Resource Center Matamoros, a migrant aid group in the Mexican border city, and, speaking in broken Spanish, said they were looking for volunteer work. Security footage shared with The New York Times shows the pair standing on the sidewalk in shorts and flip-flops as they talked via speakerphone with Gaby Zavala, the center’s founder. After about half an hour, they left. Zavala didn’t know it yet, but the men were not volunteers. They were provocateurs building an