It was three o'clock in the morning. Along a deserted pavement of Riverside Drive strode briskly a young man whose square-set shoulders and erect poise suggested a military training. His coat, thrown carelessly open to the cold night wind, displayed an expanse of white indicative of evening dress. As he walked his heels clicked sharply on the concrete with the forceful firm tread of the type which does things quickly and decisively. The intense stillness of the early morning hours carried the sound in little staccato beats that could be heard blocks away. A few yards behind him, moving furtively and noiselessly, almost as if he had been shod with rubber, crept another figure, that of a stocky, broad-shouldered man, who despite his bulk and weight moved silently and swiftly through the night, a soft brown hat drawn low over his eyes as if he desired to avoid recognition.