“We have both tried, and we have both failed. You have remarked more than once since I have been in the room that you had all the evidence which you require. Surely you will not withhold it any longer.”
“Any delay in arresting the assassin,” I observed, “might give him time to perpetrate some fresh atrocity.”
Thus pressed by us all, Holmes showed signs of irresolution. He continued to walk up and down the room with his head sunk on his chest and his brows drawn down, as was his habit when lost in thought.
“There will be no more murders,” he said at last, stopping abruptly and facing us. “You can put that consideration out of the question. You have asked me if I know the name of the assassin. I do. The mere knowing of his name is a small thing, however, compared with the power of laying our hands upon him. This I expect very shortly to do. I have good hopes of managing it through my own arrangements; but it is a thing which needs delicate handling, for we have a shrewd and desperate man to deal with, who is supported, as I have had occasion to prove, by another who is as clever as himself. As long as this man has no idea that anyone can have a clue there is some chance of securing him; but if he had the slightest suspicion, he would change his name, and vanish in an instant among the four million inhabitants of this great city. Without meaning to hurt either of your feelings, I am bound to say that I consider these men to be more than a match for the official force, and that is why I have not asked your assistance. If I fail I shall, of course, incur all the blame due to this omission; but that I am prepared for. At present I am ready to promise that the instant that I can communicate with you without endangering my own combinations, I shall do so.”

A Study in Scarlet Chapter 7