Terror and the Mexico border: How big a threat?

From Christian Science Monitor:

Islamic militants purchase a nuclear device from a sympathetic official in Pakistan and ship the weapon on the flip side of a drug trafficking route through West Africa to South America. Next, the package is smuggled north to the United States-Mexico border.

Although this sounds like the plot line of a spy thriller, it is a scenario laid out in an online magazine produced by the Islamic State, the apocalyptic Syria-based terror group also known as ISIS.