The pre-dawn call by U.S. border agents to their Canadian counterparts was unexpected: A group of nine people, most of them children, were getting ready to enter Canada on foot.

On Feb. 3 at 6:16 a.m., when the group was spotted, the border between Alberta and Montana was stark and unforgiving, covered in snow, and enveloped in darkness with a temperature of minus 17 degrees Fahrenheit.

Grainy night-vision images captured by Canadian border cameras showed two little girls in pink winter gear holding an adult’s hand as they struggled through the snow. More children followed in a line. Another adult was dragging two suitcases.

The prompt intervention by the Royal Canadian Mounted Police team that found the group was a direct result of increased border presence across the expansive frontier between the United States and Canada. The border, spanning 5,525 miles, is the world’s longest.