Tom Clancy had a plot in one of his books where a Japanese commercial pilot flew a plane into the Capitol building, killing the President and most of Congress. then we had 9/11.
Stephen Coontz has a plot in the book "Liberty" where nuclear weapons are smuggled into the country in cargo containers.
Millions of containers come into this country every day, the vast majority uninspected. Nuclear weapons are missing from some of the countries in the former Soviet Union.
So yes, it's possible. One nuclear weapon detonated in New York harbor would wipe out a signifcant portion of our refining capacity, the financial district of New York, all of the docking facilities on both sides of the river, and maybe 250,000 people or more. It would also cause every ship not unloaded, no matter what port, to be sent far off shore until inspection facilities could be set up, thus desroying our import market. Of course, that would put Walmart completely out of business, maybe not a bad thing ;-)
As far as i can see, nothing is being done to address this serious threat.
This could happen today.
The most likely culprit would of course be Islamic terrorists, although the inept recent terror plots in New York and New Jersey seem to rule out "homegrown" terrorists, and of course, no intervention by those in the US would be required.
A "false flag" operation would also be possible, but I would only be speculating as to what nation would do that.
Meanwhile, everyone is up in arms over illegal Mexicans. I think our priorities vis-a-vis terrorism are a little screwed up.