He has been called many things, but Harry Potter has never before been accused of promoting vandalism. Yet the young wizard looks to be the No 1 suspect in a grafitti spree which took place last week in Wauwatosa, a suburb of Milwaukee, Wisconsin.

According to the Chicago Tribune, residents of the town, population 45,000, awoke to discover that 80 stop signs had been defaced to read "Stop Voldemort" – Potter's nemesis, to the uninitiated. The vandal struck just before last week's opening of Harry Potter and the Half-Blood Prince, the sixth film in the series.

So far local police do not have any clear leads, though Captain Jeff Sutter said the handwriting on each sign, applied in permanent marker, looked very similar. The dark lord's name has now been removed from every one, but a number will have to replaced at a cost of $80 (£48) each.

Police were initially hamstrung in their investigations due to some confusion over who Voldemort actually was, until a policeman's son helpfully explained matters. "It took a seven-year-old to tell us," Sutter said. "We thought it was just some type of new tagger."