The EU police agency, Europol on Friday warned against more Islamic State attacks in the 28-nation bloc, with several dozen militants already in place and more possibly arriving.
Europol said the most likely forms of attack would be mass shootings and suicide bombings, like those seen in Paris and Brussels.
"If IS is defeated or severely weakened in Syria/Iraq by the coalition forces, there may be an increased rate in the return of foreign fighters and their families from the region to the EU or to other conflict areas," Europol said in a statement.
The report said that car bombs and kidnappings, common in Syria, could also emerge as tactics in Europe. While protected sites such as power grids and nuclear power stations were not seen as top targets.
Europol director Rob Wainwright said EU states had stepped up their security cooperation in the wake of IS attacks in the last couple of years, allowing more plots to be thwarted.
"Nevertheless," he said, "Today's report shows that the threat is still high and includes diverse components which can be only tackled by even better collaboration."