They had covered perhaps a hundred and fifty feet or so, and ahead of them could just make out the dark bank of the river. Suddenly they saw a man appear around the corner of the building, running toward them. Bob and Hugh crouched against the brick wall and waited for him to come near. All at once Bob recognized the stranger and started forward. "Karl," he cried. The man halted. "Where are you going?" asked Bob. "Where are father and the others?" "Down by the river," replied Karl and once more broke into a run. A moment later he was lost to sight in the darkness. "Seems to me he's in an awful hurry," remarked Hugh. "Father had probably sent him on an errand," said Bob. "Let's hurry and see if we can't find father and Sergeant Riley." "Who do you think shot Lena?" asked Hugh. "I don't know. We'd better not talk here now." "Do you suppose it could have been the fake detective?" said Hugh regardless of Bob's advice. "I don't know, but I don't see why he should shoot one of his own gang." "He blew up Mr. Wernberg though." "I know it, but I can't understand it, and as I said I don't think we ought to talk here." They proceeded in silence. Both boys were eager to join the others and they wondered what they could be doing down by the river. Perhaps they had captured the plotter and had dispatched Karl for rope or handcuffs to secure him. At any rate nothing suspicious had happened since the shots had first been heard. The boys had progressed but a short distance further, when suddenly a great tongue of flame shot heavenward between them and the river. An ear-splitting detonation followed, and the very earth was rocked by an enormous explosion. Both boys were thrown violently to the ground by the force of it, while showers of earth, bricks, and material of all kinds pelted down all about them. A moment later the boys were on their feet, still partly stunned and undecided as to whether they should run or not. "There may be another one coming," warned Hugh. While they hesitated a man suddenly appeared running swiftly away from the direction of the explosion. "Hey there!" challenged Bob. "Who are you?" For answer there came the flash of a revolver and a pane of glass in the window close beside the boys' heads was shattered. "Stop!" shouted Bob at the top of his voice and regardless of danger he started in pursuit of the fleeing man. Hugh was not to be left behind at such a time and together they raced after the fugitive. Suddenly he stopped, raised his right arm, and hurled his revolver. It struck the ground directly in front of Hugh, spun around a number of times and hit him a sharp blow on his shin bone as it caromed. "Let it alone," cried Bob. "It must be empty."