Mexico exports terrorists?

There are people in the US who believe that ISIS (or ISIL, aka Islamic State) terrorists are about to or have infiltrated the US from Mexico, amongst them Texas Governor Rick Perry. The Department of Homeland Security isn't amongst them.

For the story, see http://www.nytimes.com/2014/09/16/us/us-pushes-back-against-warnings-that-isis-plans-to-enter-from-mexico.html?emc=edit_tnt_20140915&nlid=20064986&tntemail0=y&_r=0