The US Air Force is funding work to let drones tag suspects or cars with a spray that gives them a distinct spectral signature, making them easy to track

ON A dusty road in northern Pakistan, a nondescript vehicle rounds a corner. Fifty metres overhead, a tiny drone buzzes unseen, spraying a fine mist across the vehicle's roof as it passes below. The vehicle is now tagged, and can be tracked from many kilometres away by an infrared scanner on a larger drone.

This scenario may soon be played out now that Voxtel, a firm in Beaverton, Oregon, has won a US Air Force contract to develop a drone-based tagging system. Voxtel makes tagging materials – taggants – that can be used to discreetly label vehicles carrying smuggled goods, or people who are involved in civil disobedience or attempting to cross international borders illegally.

Interest in tagging technology has been driven in part by growing pressure on the White House over civilian deaths in US drone attacks. During a recent visit to Pakistan, US Secretary of State John Kerry stated that drone strikes there wi.... Tagging by drones would allow people to be tracked for subsequent arrest.

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