New Delhi, Dec. 12 -- The oil tanker was navigating near the coast of Guyana recently when its location transponder showed it starting to zigzag. It was a seemingly improbable maneuver and the latest digital clue that the ship, the Skipper, was trying to obscure its whereabouts and the valuable cargo stored inside its hull: tens of millions of dollars' worth of illicit crude oil.

On Wednesday, U.S. commandos fast-roping from helicopters seized the 332-meter (1,090-feet) ship - not where it appeared to be navigating on ship tracking platforms but some 360 nautical miles to the northwest, near the coast of Venezuela.

The seizure marked a dramatic escalation in President Donald Trump's campaign to pressure strongman Nicolas Maduro by cutti...