And any competent wilderness-type people could bring a few hundred pounds of cargo across the northwest border with British Columbia. There are a few camera monitors and other types of surveillance, but nothing that couldn't be defeated. After all, it is foggy much of the time now, and snow covers tracks, and animals confuse infrared sensors. That mission would be so easy for special ops types it wouldn't even qualify as an difficult exercise. If a snowmobile or ATV met the intruders at a pre-arranged place for a cargo hand-off the intruders could then disappear back over the border, undetected.
If the payload were shielded cesium, cobalt, or some other special source, the next step would be to transport the material to an area of legitimate isotope sources (such as a dense medical park) for weapon assembly and detonation. The presence of other radioactive sources within 100 yards or so would considerably slow detection efforts.
I'm really surprised a scenario like this hasn't already occurred.