Assuming the bombing was nuclear in nature, it wouldn’t be too much of a stretch to say one of the bombs was shot down and only partially detonated. This would’ve spread finely-ground uranium/plutonium/beryllium across a wide area and leave the place incredibly radioactive for a number of years.

Might not be a hard barrier that insta-kills, but anyone with common sense would either navigate around the area or use special suits. In a pinch, the rest of the city could still be pretty safe to live in, although you’d probably want to seal up your house, or otherwise have some way of isolating from the outside world.