PARIS (AP) — Explosives that mimic those used in Syria and Iraq are a leading threat to the European Union — including weaponized off-the-shelf drones — along with returning fighters and would-be jihadis blocked from traveling to the war zone, according to a report Thursday from Europe's top law-enforcement organization.

In a report summarizing trends from 2016 and looking ahead, Europol says Islamic extremists bent on attacking Europe are trending younger, and more of them are female than ever before.

Arrests for jihadi activities rose last year for the third year in a row: from 395 in 2014 to 718 in 2016. Nearly a third of those arrested were 25 or younger.

The report says governments are paying close attention to the use of drone explosives by the Islamic State group in Iraq, for fear that homegrown extremists will replicate the weapons or others used to deadly effect. The bomber who struck the Ariana Grande concert in Manchester, England, used a backpack bomb apparently packed with bearings and other small pieces of metal — an echo of a deadly technique used in attacks by al-Qaida and Islamic State extremists.

"The kind of attacks that ISIS have used in the conflict zone, including car bombs perhaps and others, if that technical capability is known within the organization then clearly there's potential for that to be transferred into a European scenario," Europol chief Rob Wainwright told The Associated Press, "although one shouldn't underestimate, either, the difficulty in doing that on a consistent basis."

Wainwright said many Europeans have left the Islamic State group after growing disenchanted with life under war, if not the brutality of the extremists themselves. The concern, however, is how to distinguish them from others who are returning clandestinely to form new networks.

That threat is compounded by Europeans inspired by the group to carry out attacks — some of them radicalized quickly and under the radar.

"It's a reflection of the very serious threat that we face in Europe and a reflection of the fact that I'm afraid we can't get that threat down to zero," Wainwright said.