In total, 30 people have been arrested in an investigation into a wave of violence across Zuid-Holland province last October and November, which police now believe may be connected to the theft of a consignment of cocaine.

The Hague, Alphen aan den Rijn, Delft and Leiden were among the 17 locations targeted by a string of explosions, shootings, arson attempts and threats, the public prosecution service said in a statement on Wednesday.

Officials believe a 1,400-kilo shipment of cocaine that arrived in Belgium in August may be at the heart of the violence. They suspect the drugs were taken by a rival gang and brought to the Netherlands, where the cocaine was divided up among a larger group.

An investigation into the victims of the attacks indicated that it was not they, but their relatives and friends who may have been linked to the cocaine theft, the department said.

In total, 30 people have now been arrested over the past six months, most of whom were responsible for carrying out the actual attacks and were unaware that stolen cocaine was involved.

The investigation into those who commissioned the attacks is continuing.