Infant and toddler found alone in Arizona desert

A 4-month-old infant and an 18-month-old toddler were found in Arizona's Sonoran Desert on Friday, according to a U.S. Customs and Border Patrol agent.

The young children were left in the desert, which is located in the south of Arizona and borders Mexico, by smugglers "to die," according to a tweet from John R. Modlin, chief patrol agent of the U.S. Border Patrol’s Tucson Sector.

"This is cruelty," he said in the tweet. "And it is gut-wrenching. I commend our agents for their quick response to this dreadful incident and to every incident in which migrant lives are at stake."

The 4-month-old infant was found unresponsive, but patrol agents were able to revive her, according to Modlin.

"The heartlessness of smugglers cannot be underestimated," he said.

Earlier this week, two children died and a baby was in critical condition after being pulled from the Rio Grande during separate attempts to cross into the United States from Mexico.

The tragic incidents come amid a recent decline in arrests of undocumented migrants crossing the U.S. southern border after reaching an all-time high in May.

The drop comes amid an increase in U.S. assisted arrests of smugglers in Central America, according to internal briefing materials obtained by NBC News.

Agents from CBP and the Immigration and Customs Enforcement’s Homeland Security Investigations, including some operating in Central American countries, were responsible for the arrests of an average of 240 smugglers in a single week in June alone, according to the materials, prepared by the Department of Homeland Security for a White House briefing.