“I was on a ride-along with the US Border Patrol in McAllen, Texas, last week,” he said.

“[It’s] in the Rio Grande Valley, the most heavily trafficked part of the entire US-Mexico border for undocumented immigration. I was with them from afternoon into evening.”

Over the years, Moore had become familiar with both the routes over this border and the way different people reacted: how those who crossed individually, usually men, would run and try to hide from the border patrols. Many families, tired from their often weeks-long journey, would seek to surrender almost immediately to the officers whom they encountered.

Night had fallen and Moore was waiting in the scrub by the riverside for rafts carrying undocumented migrants, when the sound of four boats and a dozen people became audible. Those crossing were arrested almost immediately by border officers.

And unlike those in the boat, Moore knew what was likely to happen.

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“I have photographed many immigrant families seeking political asylum at the border over the last few years.

“I am sure that most of these families had no idea of the new US policy to separate children from their parents during the immigration court proceedings.

“I knew, however, what would happen to many of them next – separation – after they were taken away, so it was difficult for me to witness.

“The agents grouped everyone together,” he recalled.

“[They] looked over their documents and noted their names and home countries.”

Each of the adults was frisked before being loaded into a van to take them to a processing centre, he said.

“One of the last in line was the mother with her daughter. I had spoken with her briefly and she said she was from Honduras and that she and her two-year-old had been travelling for a month to reach the US border.

“The officer asked her to put the little girl on the ground while the mother was searched. Once on the ground the young girl immediately started crying.

“As a photojournalist covering this story for years, and as the father of three children, including a toddler, it was personally very hard for me to photograph. Very quickly the two were then bundled into the transport van, and it was over. I had to take a few deep breaths, as it was emotional for me to see.”

In the context of the debate that has convulsed US politics since the Trump administration’s policy of separating children from their parents at the border has been thrust into the spotlight – attracting criticism even from high profile Republicans and former first lady Laura Bush – Moore knew what he was seeing would have resonance.

“I knew that this image was important. There was no way for me to know just how much so,” he said, in the midst of back-to-back interviews with US networks and other media outlets.

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“For me, it’s both gratifying to make photographs that matter, and saddening to witness. As a photojournalist who has worked in difficult disaster and conflict zones for many years, that conflict of emotion never really goes away.”

For now, however, Moore would like to know what has happened to the little girl and her mother.

“US Customs and Border Protection have not been forthcoming on their status so far. I would love to know where they are and how they are doing.”