Terrorist Threat at Our Southern Border

Since 9/11, we've heard periodic rumours about terrorists entering our country via the porous southern border. So far, this has never happened.  To my knowledge, authorities have uncovered no known plot to infiltrate terrorists into the U.S. via Mexico.  Why? 

• Just flying into the country had been easy enough.  (Not so now, though.)
• Mexican gangs have no good reason to cooperate with terrorists.  They are in the money-making business.
• Mexico is not a good place for Islamic terrorists to blend in.
• Too much could go wrong: being exploited by smugglers, being captured by US law enforcement, etc.

In recent years, we've heard of one crazy plot by an Iranian-American used car salesman to kill the Saudi Ambassador in DC by contracting the Mexican Zetas. He traveled to Mexico to do this, but only made contact with a DEA informant, who set him up for a sting.  Would the Zetas have actually tried to do this?  I doubt it.  They are under enough pressure as it is.

Hard to believe from the news coverage, but it is actually MUCH harder to cross the southern border illegally than it used to be.  Before 9/11, apprehensions were one million a year.   Recently it has been in the low 100,000s.  This year there's been a spike that might bring us closer to 500,000.  Most of the surge has been in Texas's Rio Grande valley, which didn't used to be that heavy.  (The Tuscon area had been the worst.)