A North Korean nuke would be unlikely in Long Beach. China's economy depends too much on trade through Long Beach, and IIRC, many of those facilities actually belong to China. I am confident Chinese spies would know if NK was responsible and would not be inclined to tolerate continuation of the NK regime, and the NK regime knows it.
A NK nuke, remarketed through Al Quaeda or JI is a bit more possible, but China would still be likely to interfere in order to defend its best interests. Also, it would be somewhat more difficult (but my no means impossible) for AQ etc. to be able to load, place and ship the bomb container.
I think Pakistan, so far, is rational enough to want to prevent its nukes from actually being used on US soil. I am pretty sure we know the 'signature' of their nuke fuel, and they know we would trace a blast back to them, with severe consequences - even from a Dem admin. Among other things India would be given whatever they wanted and carte blanche to use it.
Then there's Iran. I'm not sure we could know their bomb if it hit us. I'm not sure they have the rational thought process to prevent them from using it. And I'm not sure anyone else has the knowledge and ability to control them either.

The study just considered the possibility of a blast without really wondering where the nuke might have come from.

The reasoning out that you do, though, does show how difficult such things would be to determine . . .

Think Dirty Bomb!

Here's another thought and a lot easier to do.

AQ or other Islamofascist terror cells slip over US Southern Border into Southern California with the components for a dirty bomb.

Other cell members take truck driver training at any one of the many truck terminals in the area. Obtain shipping container from the area e.g. Riverside/San Bernardino Counties.

Drive the container into the Port of Long Beach and detonate it in the port. The economic impact of such an attack on the US would be $40 BILLON dollars.

RBT

Think Dirty Bomb!

Here's another thought and a lot easier to do.

AQ or other Islamofascist terror cells slip over US Southern Border into Southern California with the components for a dirty bomb.

Other cell members take truck driver training at any one of the many truck terminals in the area. Obtain shipping container from the area e.g. Riverside/San Bernardino Counties.

Drive the container into the Port of Long Beach and detonate it in the port. The economic impact of such an attack on the US would be $40 BILLON dollars.

RBT

I think we, through the now defunt UN inspections, have samples from the Iranian nuclear work. I'm certain there are American and Isreali covert efforts to acquire more samples for identification purposes.

A nuclear attack on American soil would result in a nuclear response - hopefully of the 'strategic' flavor.