The email arrived from an unknown source on Sunday morning, saying that “something would happen,” and officials should be prepared for the event of a school attack.
Police searched the Walsum district school, where two voting stations had been located for Sunday's national election, but found nothing suspicious, they said.
Nevertheless, authorities decided to keep the comprehensive school, and a neighbouring college preparatory school, the Kopernikus Gymnasium, closed for the first day of the week.
Experts are now trying to root out the email sender, and said they believe it was composed by one or more students. According to the Duisburg police spokesperson, school threats are the “modern way to get out of tests.”
In mid-September an 18-year-old student attacked and Ansbach high school with five Molotov cocktails, knives and an axe. One teacher and nine students were injured – two girls severely.
German authorities have been on high alert since March, when 17-year-old Tim Kretschmer killed 15 people and himself at a school in Winnenden, Baden-Württemberg.