Remeber the guy they found in Italy on his way to
Canada with his living accomodations in a container.
It was likely a test!
Remember, these guys really believe in dry runs, and getting the logistics nailed down. The 9-11 team did repeated dry runs.
A man might be sent along with a WMD and a hand GPS unit to detonate it when the container arrives in the target port, or as a backup to detonate it if discovered.
Even without a nuke, you could fit a LOT of Sarin in a 40X10X8 box. Or the "mother of all dirty nukes" built around a 40' long HE bomb.