Usage patterns
〈人〉の仕業だit's the work/doing of someone
This graffiti is the work of a kid from the neighborhood.
〈人〉の仕業に違いないit must be someone's doing
The candy disappearing from the fridge must be my little brother's doing.
何者かの仕業the doing of some unknown person
The scratch on the wall seems to be someone unknown's doing.