A coordinated anti-terror operation saw officials swoop in to arrest six people for allegedly raising funds and recruiting fighters for ISIS, authorities in Spain, the United Kingdom and Germany said in statements on Wednesday.
According to Spanish authorities, four of the six were detained in Mallorca, while the others were taken into custody in the UK and Germany.
Spanish National Police said the investigation linking the group to ISIS started in 2015 when a website with several terrorist propaganda videos was discovered.
During the course of the two-year probe, authorities identified a UK-based Salafist imam who was allegedly orchestrating the production of online extremist propaganda.
The 44-year-old imam, whose identity has not been publicly disclosed because of ongoing proceedings, was detained on terrorist offenses at an address in Sparkhill, Birmingham, by the West Midlands Counter Terrorism Unit at the request of Spanish authorities.
According to Spanish police, the man is believed to be the group's spiritual leader and is known to have traveled to Mallorca on multiple occasions to set up the group that included the four who were arrested in Spain on Wednesday.
They added that the imam's public speeches had already put him on the radar of British and European security services.
The man taken into custody in Germany is also believed to have been working with the group and had taken part in several of their propaganda videos, according to the Spanish National Police.
Authorities took action across Europe on Wednesday after the investigation appeared to indicate that the group's following was growing as a result of increased social media activity and secret weekly meetings at which their extremist ideology was being disseminated.
In the wake of the recent terror attacks in the UK, British Prime Minister Theresa May has called on internet companies to do more to tackle the prevalence of extremist material on the web
and established a joint campaign with French president Emmanuel Macron to battle online radicalization.
The arrests on Wednesday are the fruit of a two-year multinational operation, but some have questioned how much access to European security agencies and intelligence databases the UK will receive it leaves the European Union.