PHOENIX (AP) — The Border Patrol says one of its agents rescued an infant and a toddler who were left alone by migrant smugglers in western Arizona’s Organ Pipe Cactus National Monument.

A migrant in a group of border crossers arrested Thursday west of the Lukeville, Arizona, port of entry alerted an agent to the children’s location.

An 18-month-old was subsequently found crying and a 4-month-old was discovered face down and unresponsive. Both have received medical attention at a hospital and were released back into Border Patrol custody.

“Yesterday smugglers left two young children—an infant and a toddler— in the Sonoran Desert to die,” Tucson Sector Border Patrol Chief John Modlin said in a statement. “This is not just another example of smugglers exploiting migrants for money. This is cruelty.”

It was not immediately clear if the smugglers blamed for abandoning the children were among those arrested. Authorities did not release any details about the children, including their genders, what country they came from and the identities of their parents or guardians.

Organ Pipe Cactus National Monument stretches along the Mexican border in southwestern Arizona, a harsh, dry landscape studded with towering cactuses and other desert flora. It’s about 130 miles (209 kilometers) southwest of Phoenix.

Because of its remoteness, the 517- square-mile (1,340 square kilometer) park is a favored crossing area for some smugglers. he human remains of suspected border crossers are often in the area.

While the high temperatures at Organ Pipe Cactus National Monument regularly soar into the triple digits during the summer, they topped out Thursday in the 90s amid cooler monsoon weather.

The agent who found the children began first aid on the infant while waiting for medical technicians from the Border Patrol and the National Park Service to arrive.

An ambulance took the children to a hospital for additional medical attention. 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 migrant children picked up in the U.S. without family.

The Border Patrol’s Tucson Sector says it has seen a 12% increase in unaccompanied children being smuggled across the border over the past fiscal year.