It was Gavegan.

Larry realized that he had been shrewdly trapped, that resistance would serve no end, and the next moment handcuffs were upon his wrists.
"Well, Brainard," gloated Gavegan, "we've landed you at last!" "So it seems, Gavegan." "You thought you was damned clever, but I guess you know now you ain't one, two, three!" "Oh, I knew how clever you are, Gavegan," Larry responded dryly, "and that you'd get me sooner or later if I hung around." As a matter of fact Larry's capture, which was as unspectacular as his escape had been strenuous, was the consequence of no cleverness at all.

Larry had said to Barney Palmer the night before that he knew who Barney's sucker was; and Barney had passed this information along to Chief Barlow.

"Follow every clue; luck may be with you and one of the clues may turn up what you want":--this is in substance an unwritten rule of routine procedure which effects those magnificent police solutions which are presented as more mysterious than the original mystery--for it is well for the public to believe that its police officers are unfailingly more clever than its criminals.

Barlow had done some routine thinking: if Larry Brainard knew Dick Sherwood was the sucker, then watching Dick Sherwood might possibly reveal the whereabouts of Larry Brainard.