Translation:There was no doubt as to who was responsible.
Oh I didn't see that... However 'som' could still not be skipped. This cannot be explained in a very simple way, but I can give you some examples:
Det er han som løper = It is he who runs
Vi vet ikke hvem som løper = We don't know who runs
Alternatively the sentences can be rephrased.
Det var ingen tvil om hvem det var som var ansvarlig.
There was not doubt as to who it was that was responsible.