We had turned the corner of Z Street and were approaching the house when a man walking in the opposite direction eyed us suspiciously, turned, and followed us a step or two.

"Kennedy!" he exclaimed.

If a fourteen-inch gun had exploded behind us I could not have been more startled.Here, in spite of all our haste and secrecy we were followed, watched, and beaten.

Craig wheeled about suddenly.Then he took the man by the arm.

"Come," he said quickly, and we three dove into the shadow of an alley.

As we paused, Kennedy was the first to speak."By Jove, Walter, it's Burke of the Secret Service," he exclaimed.

"Good," repeated the man with some satisfaction."I see that you still have that memory for faces." He was evidently referring to our experiences together some months before with the portrait parle and identification in the counterfeiting case which Craig cleared up for him.

For a moment or two Burke and Kennedy spoke in whispers.Under the dim light from the street I could see Kennedy's face intent and working with excitement.

"No wonder the War Department is a blaze of lights," he exclaimed as we moved out of the shadow again, leaving the Secret Service man.

"Burke, I had no idea when I took up this case that I should be doing my country a service also.We must succeed at any hazard.

The moment you hear a pistol shot, Burke, we shall need you.Force the door if it is not already open.You were right as to the street but not the number.It is that house over there.Come on, Walter."We mounted the low steps of the house and a negress answered the bell."Is Mr.Gonzales in?" asked Kennedy.

The hallway into which we were admitted was dark but it opened into a sitting-room, where a dim light was burning behind the thick portieres.Without a word the negress ushered us into this room, which was otherwise empty.

"Tell him Mr.Montez is here," added Craig as we sat down.

The negress disappeared upstairs, and in a few minutes returned with the message that he would be down directly.

No sooner had the shuffle of her footsteps died away than Kennedy was on his feet, listening intently at the door.There was no sound.He took a chair and tiptoed out into the dark hall with it.

Turning it upside down he placed it at the foot of the stairs with the four legs pointing obliquely up.Then he drew me into a corner with him.

How long we waited I cannot say.The next I knew was a muffled step on the landing above, then the tread on the stairs.

A crash and a deep volley of oaths in French followed as the man pitched headlong over the chair on the dark steps.

Kennedy whipped out his revolver and fired point-blank at the prostrate figure.I do not know what the ethics are of firing on a man when he is down, nor did I have time to stop to think.

Craig grasped my arm and pulled me toward the door.A sickening odour seemed to pervade the air.Upstairs there was shouting and banging of doors.

"Closer, Walter," he muttered, "closer to the door, and open it a little, or we shall both be suffocated.It was the Secret Service gun I shot off - the pistol that shoots stupefying gas from its vapour-filled cartridges and enables you to put a criminal out of commission without killing him.A pull of the trigger, the cap explodes, the gunpowder and the force of the explosion unite some capsicum and lycopodium, producing the blinding, suffocating vapour whose terrible effect you see.Here, you upstairs," he shouted, "advance an inch or so much as show your heads over the rail and I pump a shot at you, too.Walter, take the gun yourself.

Fire at a move from them.I think the gases have cleared away enough now.I must get him before he recovers consciousness.

A tap at the door came, and without taking my eyes off the stairs I opened it.Burke slid in and gulped at the nauseous atmosphere.