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Agents believe that the aircraft are being used for civilian purposes – to ferry offshore oil workers around – but at least one of them is equipped with sensitive night vision and autopilot capabilities, which are subject to strict U.S. controls.

Both helicopters were bought from a Mexican company and made pit stops in North Texas before heading to Italy, then Iran, documents show.

But investigators have found no evidence that any Americans had a hand in the final leg of the helicopters' trip, so no criminal charges have been filed.

The investigation is an example of the difficulties U.S. authorities encounter enforcing export control laws meant to keep sought-after American technology and hardware out of the hands of countries such as Iran. The country has been subject to sanctions for more than a decade because it sponsors international Islamic terrorism.