Interesting suitcase nukes article here-
He says they're probably around only 1 kiloton strength (Hiroshima was 10 kt), but can still do a lot of damage. For example they can flatten a city centre out to about half a mile from ground zero, killing everybody in that radius with the fireball and blast wave, and the fallout will kill more for several miles downwind. He estimates a total of 20.ooo people will die.
Because suitcase nukes are so small and light he says they can be easily flown over a city in a light aircraft and dropped out.
Just to add my two cents- the muslim droppers would be prepared to get caught in the blast themselves and die as "martyrs", but they might opt for a parachute release instead, dropping the bomb from high altitude, then getting away at full throttle before it hit the ground.
Alternatively they could smuggle the bomb ashore by rubber boat from a submarine or innocent-looking merchant ship, set its time fuse and then get back to the sub or ship.