German authorities are investigating what a Berlin restaurant owner was doing with a deck of playing cards marked with a radioactive substance.

On Tuesday, German police announced that a routine inspection earlier this month at a waste treatment facility in Berlin-Lichtenberg had turned up playing cards dosed with iodine 125, a radioisotope used in medical procedures.

The radioactive trail led police to a garbage truck, and an investigation into the route taken by this truck and other garbage found with the cards eventually led police to an unidentified 41-year-old restaurant owner. Police said they found a total of 13 irradiated playing cards, all of which were “the same color.”

The Berlin police noted that they detected higher than usual radioactivity in the suspect’s restaurant, as well as in some adjoining facilities, including an Asian market, a karaoke bar and an apartment.