An immigrant in a group of border crossers apprehended Thursday west of the Lukeville, Arizona port of entry notified an agent of the children’s location. Subsequently, an 18-month-old was found crying and a 4-month-old was found face down and unresponsive. Both were treated at a hospital and released back into Border Patrol custody. “Yesterday smugglers left two young children – an infant and a toddler – in the Sonoran Desert to die,” Tucson Border Patrol Chief John Modlin said. “This is not just another example of smugglers exploiting migrants for money. This is cruelty.” It was not immediately clear if the smugglers accused of abandoning the children were among those arrested. Authorities have not released any details about the children, including their gender, country of origin and the identity of their parents or guardians. Organ Pipe Cactus National Monument stretches along the Mexican border in southwestern Arizona, a harsh, dry landscape filled with towering cacti and other desert flora. It is about 130 miles (210 kilometers) southwest of Phoenix. Because of its remoteness, the 517-square-mile (1,340 sq km) park is a favorable transit area for some smugglers. The human remains of border crossing suspects are often found in the area. While high temperatures at Organ Pipe Cactus National Monument regularly soar into the triple digits (above 37.7 degrees C) during the summer, they topped the 90s Thursday (above 32.2 C) amid a cooler monsoon . The agent who found the children began first aid on the infant while waiting for Border Patrol and National Park Service medical technicians to arrive. An ambulance transported the children to the hospital for additional medical care. The children were later returned to Border Patrol officials, who were trying to urgently place them with the Office of Refugee Resettlement, which oversees the care of immigrant children who have been taken to the U.S. without family. The Tucson Division of the Border Patrol says it has seen a 12 percent increase in unaccompanied children being smuggled across the border in the past fiscal year. Our Morning Update and Afternoon Update newsletters are written by Globe editors, giving you a concise summary of the day’s most important headlines. Sign up today.