RM investigation after letter sent to Asian nanny is defaced

[fishtank]

However, if it is an RM employee, it will be difficult.

To do this in a mailcentre or delivery office you would need to remove the item without being spotted and take it somewhere where there are no cameras or no chance of anyone seeing you,then after writing the message you would have to bring it back and replace it,again without being spotted.
Although it's possible it seems like a big risk to take to indulge in some random act of racial hatred with someone you don't know and whose reaction you wont even be aware of.
If it was either the collection OPG or the delivery OPG they would surely have known they would be suspects.
If it was a Royal Mail employee as well as being a bigot they were incredibly stupid.

[Wessex]

I would also suggest that it is the contents of the letter perhaps, and not just the name of the recipient that provoked the insults .I would make my first port of call the mail room of the sender.
Particularly, if they had casual staff working there on that day.