if they really have the bomb that can work, they will absolutely send through conventional routine that have to deal with the custom, there are many ways:
such as transport them to the badly defended Canada or Mexico and penetrate the border through means like parachute, light-model aircraft in low altitude to penetrate the airforce national guard, or through things like a aircraft with remote control capacity, and so on.
the point is if 20 bombs which require short term perodic maintainance placed in the most critical metropolitan:
I guess:
D.C
New York
Chicago
L.A
San Francisco
Phalidalphia
Boston
Detroit
Houston
Maimi
Denver
Las vegas
st Louis
Dallas
San Diego
Cleveland
New Orlean
Atlanta
Tampa
Seattle
the FBI just need to check the record of local hospital or nuclear physics labs or University, or any way that can have access nuke-detonating radiological material through commercial means to determine if the 20 bombs can be properly maintained.