4 Dutch Morrocans Arrested who planned attack on Turkish Consulate

The Police have arrested 4 Dutch Moroccans whom are suspected of planning an attack on the Turkish Consulate in Rotterdam.

 The arrests were based on fragments of recordings from autumn 2016 by the AIVD General Security Services.   Observation: why did it take a year and a half to arrest them?

https://www.ad.nl/rotterdam/vier-mannen-opgepakt-wegens-plan-aanslag-turkse-consulaat-rotterdam~a89d55eb/