Europe’s top cop is warning that “another attempted attack is almost certain” as the continent faces its “highest terrorist threat” since 9/11.

Rob Wainwright, the director of the European Union’s law enforcement agency Europol, told TIME “several hundred” battle-trained European jihadists were likely planning major attacks. He said his agency was supporting 50 ongoing terror investigations.

“We have 5,000 Europeans who have been radicalized by [ISIS] and have traveled to Syria and Iraq and engaged in conflict experience,” he said. “We suspect that about one-third of them have come back: That is our best guess.”

Wainwright noted that some Europeans who returned were not planning to engage in terrorist activity and others were in rehab programs or prisons.

“That still leaves, however, maybe several hundred that are potential terrorists,” he told TIME.

Wainwright said the European Football Championships this summer in France – where the top soccer players in Europe meet – represented one potential target.

He also warned that Britons could be less safe if they vote to leave the EU on June 23 in a so-called “Brexit.”

“Britain would have access to fewer capabilities in a less effective way, which at the moment plays one big part in the way Britain secures its borders and deals with the threat from terrorism and crime,” he said.

Click for more from TIME.