John Moore, a Pulitzer Prize-winning photographer for Getty Images, has been photographing immigrants crossing the U.S.-Mexico border for years. This week one of his pictures became the most visible symbol of the immigration debate in America.

“This one was tough for me. As soon as it was over, they were put into a van. I had to stop and take deep breaths,” Moore told TIME Tuesday, describing his reaction to the scene of a two-year-old Honduran girl crying as her mother was being detained in McAllen, Texas. “All I wanted to do was pick her up. But I couldn’t.”