The body of Mr Williams, 31, originally from Anglesey, was found locked in the bag in the bath of his London flat.
Det Ch Insp Jackie Sebire said "two minor components of another contributor's DNA" were found on the zip toggle and padlock.
She said it was her opinion "that a third party had been involved".
Mr Williams's death has puzzled Scotland Yard detectives who have been unable to determine a cause of death or establish whether he had locked himself into the bag.
And then it gets weird.