Authorities across Europe mounted a manhunt for those involved in a large-scale terrorist plot following the deadliest attacks to strike Spain in more than a decade: two vehicle assaults in Barcelona and the Catalan coastal town Cambrils, reports the Washington Post.

Investigators believe that at least eight people plotted the attacks, putting them at a level of sophistication comparable to major strikes in Paris and Brussels in recent years. Other more recent attacks in London, Berlin, and the southern French city of Nice were perpetrated by individuals operating largely on their own.

Spanish counterterrorism officers were scrambling to untangle the terrorist network, which involved at least four Moroccan citizens under age 25, according to intelligence officials. In addition to those four, authorities have detained three Moroccan men and a Spaniard.

In a sign that the attack could have been significantly worse, police said they believed the assailants were planning to use propane and butane canisters in an explosive assault against civilians. Instead, the gas ignited prematurely, destroying a house in Alcanar, about 100 miles southwest of Barcelona that was being used by the suspects. The explosion killed at least two people and injured 16, including police officers and firefighters investigating the site.

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