“Touched nothing, of course?” asked Bristol sharply of the officer on duty.

“Nothing, sir.  It’s just as we found it when we forced the door.”

“Why did you force the door?”

“He rung us up at the station and said that something or somebody had got into the house.  It was evident the poor gentleman’s nerve had broken down, sir.  He said he was locked in his study.  When we arrived it was all in darkness—­but we thought we heard sounds in here.”

“What sort of sounds?”

“Something crawling about!”

Bristol turned.

“Key is in the lock on the inside of the door,” he said.  “Is that where you found it?”

“Yes, sir!”

He looked across to where the brass knob of a safe gleamed dully.

“Safe locked?”

“Yes, sir.”

Professor Deeping lay half under the table, a spectacle so ghastly that I shall not attempt to describe it.

“Merciful heavens!” whispered Bristol.  “He’s nearly decapitated!”

I clutched dizzily at the mantelpiece.  It was all so utterly, incredibly horrible.  How had Deeping met his death?  The windows both were latched and the door had been locked from within!

“You searched for the murderer, of course?” asked Bristol.

“You can see, sir,” replied the officer, “that there isn’t a spot in the room where a man could hide!  And there was nobody in here when we forced the door!”

“Why!” cried my companion suddenly.  “The Professor has a chisel in his hand!”

“Yes.  I think he must have been trying to prise open that box yonder when he was attacked.”

Bristol and I looked, together, at an oblong box which lay upon the floor near the murdered man.  It was a kind of small packing case, addressed to Professor Deeping, and evidently had not been opened.

“When did this arrive?” asked Bristol.  Lester, the Professor’s man, who had entered the room, replied shakily—­

“It came by carrier, sir, just before I went out.”

“Was he expecting it?”

“I don’t think so.”

Inspector Bristol and the officer dragged the box fully into the light.  It was some three feet long by one foot square, and solidly constructed.

“It is perfectly evident,” remarked Bristol, “that the murderer stayed to search for—­”

“The key of the safe!”

“Exactly.  If the men really heard sounds here, it would appear that the assassin was still searching at that time.”

“I assure you,” the officer interrupted, “that there was no living thing in the room when we entered.”

Bristol and I looked at one another in horrified wonder.

“It’s incomprehensible!” he said.

“See if the key is in the place mentioned by the Professor, Mr. Cavanagh, whilst I break the box.”

I went to a great, open bookcase, which the frantic searcher seemed to have overlooked.  Removing the bulky “Assyrian Mythology,” there, behind the volume, lay an envelope, containing a key, and a short letter.  Not caring to approach more closely to the table and to that which lay beneath it, I was peering at the small writing, in the semi-gloom by the bookcase, when Bristol cried—­