Police in the town of Mjölby resorted to good old fashioned trickery when it came to catching the suspected thief in the act.

They added an invisible powder to the cash that would turn blue when touched with moist fingers.

The rub: if anyone handled the money, their fingers would turn blue the minute they washed their hands.

Just days after laying the trap, staff noticed a teenage intern walking around the company with both blue hands and a blue nose, unwittingly revealing himself as the culpable criminal.

While police admitted it was a long shot and an “unusual” method, they were glad to put an end to the company’s petty cash problem.

“It was a successful outcome for both the employer and perhaps also the culprit. It was good that it was discovered. When this type of theft occurs at a company, there are many people who suffer from it,” said Göran Karlsson of the Mjölby police to the local Östgöta Correspondenten newspaper.

Karlsson told the paper that while the dye wasn’t permanent; it takes a lot of effort to clean it off one’s fingers, something the teenager apparently hadn’t considered to be worth the trouble.

While the fleet-fingered thefts occurred between August and October last year, the blue-fingered intern wasn’t made to face the law until Thursday, when he admitted to the robbery of 22,000 kronor ($3,323), and was duly charged with theft.

Oliver Gee

Follow Oliver on Twitter here