In the city of Aachen, police received an email overnight making a threat against the municipal justice centre, a police spokesperson said.

The centre was evacuated and is being searched with the help of sniffer dogs.

According to dpa sources, other police stations received threatening emails that appeared to target judicial buildings.

There was a similar series of bomb threats emailed to authorities in North Rhine Westphalia in late October.

No explosives were found in that case. The investigators eventually traced the threats back to the computer of an innocent man whose device had been used by an unknown person.