No terrorist group has the capability to source the materials, technology, or knowledge necessary to construct a nuclear weapon of the type on top of our warheads or inside the WWII bombs. However, a less-sophisticated radioactive device, like a dirty bomb, is certainly within the realm of possibility.
So we're not going to see entire cities leveled, but we could very well see portions of cities contaminated with low-level radioactivity.
The bit about MS-13 transporting materiel across the border is an interesting theory, but I'm not sure if I buy it. It's one of those things that could be possible, but we'll never know. Increased border security would be nice, though
Oh yeah, as for "nuking whoever does this," good luck determining which country is "most" at fault.