The Southern Mexican border and illegal immigration's new frontier

Salvadorean migrants run to jump on a train to the border with the United States, in Lecheria, 30 km north of Mexico City, on June 1, 2010. On a visit to Washington in May, Mexican President Felipe Calderon urged the US government to repair an outmoded immigration system and do more to ensure that illegal guns do not flow across the countries' shared border.

The number of people crossing into the U.S. from Mexico illegally has dropped dramatically since peaking in 2007. But now, U.S. officials are starting to watch the flow of humans, guns and drugs at another frontier: the southern Mexican border.

The number of people crossing into the U.S. from Mexico illegally has dropped dramatically since peaking in 2007. But now, U.S. officials are starting to watch the flow of humans, guns and drugs at another frontier: the southern Mexican border.

Central Americans looking to enter the U.S. are travelling through the porous line between Mexico and Guatemala, and that's allowing the smugglers and gangs controlling those routes a chance to thrive.

Here with more on the shifting trends in immigration is Sebastian Rotella, investigative reporter with ProPublica.