NEW DELHI—A fishing boat slid into the southern Indian coastal village of Kodikkarai late one evening in mid-June, carrying a cargo from Sri Lanka: a stack of gold bars weighing more than 40 pounds.

Four men jumped off the boat and headed for waiting Jeeps. Unfortunately for them, government sleuths were there, tax officials say, nabbing the men before they could sell the gold in the nearby city of Chennai, a trading hub for the precious metal.

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