Tom Swift and His Airline Express by Victor Appleton

Like a flash Tom Swift realized that this warning had come from those daring enemies of his who were still at large-the same men, Kenny and Schlump and the two masked ones, who had kidnapped him. He could realize their rage at his escape, their anger at the foiling of their plot to blow the place up by bombs, or, if their intention was not to cause serious damage, but only confusion, during which they might rob-this, too, had been frustrated.

For a moment the sinister character of all that had taken place stunned the young inventor. The danger under which he was, came to him with a sickening realization and he sat for a moment holding the receiver in his nerveless hand.