Here is more on the issue from back in June:
Many of the children and families coming to the U.S. are fleeing violence in Central America, as shown in this report from the Department of Homeland Security, and posted by the Pew Research Center:
Another factor is Mexico's southern border:
NBC News has a subject page on the "Immigration Border Crisis" which follows the many facets of this story: http://www.nbcnews.com/storyline/immigration-border-crisis
More to come soon!