Saying that a 10-kiloton backpack nuke would kill fewer people than the 9/11 attacks seems strange. Even not airbursted, 10,000 tons of TNT-force is a big fucking explosion that would basically level a hundred-meter circle and kill tens of thousands outside that by setting the whole city centre on fire.
Imagine Hiroshima going off in Times Square on New Years. That's the ultimate fear of antiterrorist organisations. It would be to 9/11 what 9/11 was to the Paris attacks.
The only people on the planet who would ever have access to that sort of compact nuclear device, is us.
We'd pretty much have to deliberately set it up, have everyone conveniently ignore the big barrel sitting in Times Square, and then set it off, which isn't going to happen.
Assuming someone managed to get one and was smart enough to try to obscure it somewhere, that means it's now in or behind or under a building, 10 kt is a good sized blast, but you're assuming a lot shooting for that high of an effect, even the nuclear demolition plans were more along the 1~5 kt range, and the
actual smallest designs from extremely advanced nuclear powers are something like 45 kg and the only actual test of one was Redwing Yuma which fizzled and put out 190 t, though the artillery shell designs were a similar size and the prototype was shooting for 2 kt. Hardtack II had a 6 kt test of around a 23 kg device.
So, you see a bodybuilder hefting around a big suitcase with an oddly high moment of inertia in Times Square, tell someone?
Look at the chart down the page here:
http://blog.nuclearsecrecy.com/2013/12/23/kilotons-per-kilogram/Notice how the far right diagonal is 6 kt/kg? That sounds like you could make one hell of a briefcase nuke huh, but if you notice all the weapons in that range are the high yield thermonuclear weapons, and they only get that high a yield/weight ratio due to EXTREMELY high yields, as they are all huge weapons.
The W54 and such are over along the 0.01 kt/kg line, the weapon dropped on Hiroshima weighed about as much as the big armored mercedes Hitler rode in (4,400 kg) and generally you'd be doing pretty good as a terrorist group to get even a big SUV sized weapon capable of kt yields.