“I do not know how long we stood, staring stupidly at each other, but it was Lyle who was the first to recover.  He seized me by the arm and pointed excitedly into the empty box.

“‘Do you appreciate what that means?’ he cried.  ’It means that someone has been here ahead of us.  Someone has entered this house not three hours before we came, since eleven o’clock this morning.’

“‘It was the Russian servant!’ I exclaimed.

“‘The Russian servant has been under arrest at Scotland Yard,’ Lyle cried.  ’He could not have taken the letters.  Lord Arthur has been in his cot at the hospital.  That is his alibi.  There is someone else, someone we do not suspect. and that someone is the murderer.  He came back here either to obtain those letters because he knew they would convict him, or to remove something he had left here at the time of the murder, something incriminating—­the weapon, perhaps, or some personal article; a cigarette-case, a handkerchief with his name upon it, or a pair of gloves.  Whatever it was, it must have been damning evidence against him to have made him take so desperate a chance.’

“‘How do we know,’ I whispered, ‘that he is not hidden here now?’

“‘No, I’ll swear he is not,’ Lyle answered.  ’I may have bungled in some things, but I have searched this house thoroughly.  Nevertheless,’ he added, ’we must go over it again, from the cellar to the roof.  We have the real clew now, and we must forget the others and work only it.’  As he spoke he began again to search the drawing-room, turning over even the books on the tables and the music on the piano.

“‘Whoever the man is,’ he said, over his shoulder, ’we know that he has a key to the front door and a key to the letter-box.  That shows us he is either an inmate of the house or that he comes here when he wishes.  The Russian says that he was the only servant in the house.  Certainly, we have found no evidence to show that any other servant slept here.  There could be but one other person who would possess a key to the house and the letter-box—­and he lives in St. Petersburg.  At the time of the murder he was two thousand miles away.’  Lyle interrupted himself, suddenly, with a sharp cry, and turned upon me, with his eyes flashing.  ‘But was he?’ he cried.  ’Was he?  How do we know that last night he was not in London, in this very house when Zichy and Chetney met?’

“He stood, staring at me without seeing me, muttering, and arguing with himself.

“‘Don’t speak to me,’ he cried, as I ventured to interrupt him.  ’I can see it now.  It is all plain.  It was not the servant, but his master, the Russian himself, and it was he who came back for the letters!  He came back for them because he knew they would convict him.  We must find them.  We must have those letters.  If we find the one with the Russian postmark, we shall have found the murderer.’  He spoke like a madman, and as he spoke he ran around the room, with