Many operations could make an equivalent to the US MK7 60 kt bomb of the early fifties. This machine could be delivered by a light freight aircraft, like a DC-3.
From my "Effects of Nuclear Weapons" 1977, U.S.Army, pamphlet 50-3, 653 pages, (the latest data released to the public), looking at the effects of 60 kilotons, I see that the old MK7 air burst would deliver a ten psi overpressure at just over a mile radius from the burst. Even ten seconds after the explosion pressure waves would average one pound per square inch. An exposed human will achieve a velocity at one mile of about 70 feet per second, about 48 miles per hour. The wind will blow this man off his feet and then over the ground at nearly fifty miles per hour, scraping along the concrete, hitting parked cars, etc. Cars are air born in such conditions. The wind is also red hot. The area is almost completely rubble.
At 2.4 mile radius over 50% of those killed would be killed by radiological effects. At a three mile radius exposed skin will receive second degree burns. At a six mile radius the expectable total death and serious injury rate is about one in three of those within the twelve mile diameter circle.
I don't think that their is much interest in "dirty bombs" amongst the "al Queda" types. I think what I am describing is what they are working toward. They have to strike the USA in some meaningful way now instead of a totally symbolic target (like the World Trade Center). Anything less will result in merely more hostile Islamic governments, which obviously "al Queda" wishes to prevent.
i agree with your analysis. except where you say the the WTC was a symbolic target. 3000 dead is more than symbolic. and it did have actual effects we are still feeling today in manhattan as well as psychological effects throughout the country. i would hate to see a meaningful attack. but i do agree there are more terrible ways to attack which would inflict more harm. you are correct.