The investigation into the nine arsonists of City Hall in northern France was terminated because of a "procedural error", five of them, who were in custody, were released by mistake, the television channel TF1, citing informed sources, reported on Thursday.

The mistake was due to the lack of signature of the prosecutor's office at the bottom of the court document, without which the investigation and preventive measures taken are invalid, informs TF1.

According to the TV channel, the town hall in the commune of Mont-en-Baroeul was set on fire during riots in France on the night of June 28-29, three police officers were inside the building at the time of the arson. As a result, 11 people, including one minor, were later detained. Nine perpetrators were under investigation, five of them were placed in custody, and four more were placed under judicial control, the TV channel notes.