Several schools in the German city of Duisburg remained closed on Monday as a precaution after warnings that they would be targeted with criminal actions.

The messages contained "threatening and right-wing extremist statements," police in the western German state of North Rhine-Westphalia said.

What do we know about the threats?

Police said a German comprehensive school had received a letter from an unknown individual on Friday. It threatened criminal actions to take place Monday.

The same school on Sunday received another letter also containing threats, but this time linked to 13 other schools in the city.

Experts from Duisburg's State Security Service do not believe the "seriousness of the announced offenses," they added.

However, state government officials suspended in-person classes at all secondary and comprehensive schools and one grammar school in Duisburg. Some of these schools would offer online classes, officials announced.

A total of about 17,980 children at 15 comprehensive schools and two secondary schools were affected.

Other schools are set to remain open, although some are allowing parents to decide whether to send their children to classes.

The Westdeutsche Allgemeine Zeitung newspaper said decision-makers had classified the threat level as "Threat Level 2" — which would include the danger of violent attacks on multiple students or death threats — in their action plan.

The police announced a presence at the affected schools to "further minimize any possible residual risk." They said investigations into the author's identity and the background of the threat were ongoing.

Edited by: Wesley Rahn