The U.S. has been urging those governments to take steps to stem the exodus of children and warning that the U.S. will take steps to send them back promptly.

The volume of child immigrants has significantly taxed resources at the Homeland Security and Health and Human Services departments in recent months and prompted President Barack Obama to ask Congress to approve an emergency $3.7 billion spending bill to deal with the situation he has called an “urgent humanitarian crisis.”

The officials, who spoke on the condition of anonymity because they were not authorized to publicly discuss the latest trends in border arrests, said Friday it’s too soon to know what has led to drop in border arrests. They said illegal border crossings along the Mexican border tend to slow during the summer.