European arrests in terrorism cases were up sharply last year, as law enforcement moved to address a shift by jihadist groups to carry out mass casualty attacks on the continent, according to a new report.

The threat of terrorism in Europe remains on an upward trajectory, as jihadist terrorists have shifted to a strategy that seeks to inflict high numbers of deaths, a report by Europol, the European Union police agency, said.

“The attacks in Paris in January and November 2015 represented a clear shift in the intent and capability of jihadist terrorists to inflict mass casualties on urban populations,” the report said.

Europol estimated that terrorist attacks in Europe resulted in 151 fatalities in 2015, all but three in France. In 2014, Europol reported only four people were killed in terror attacks.

According to the report, 1,077 people were arrested for terrorism-related offenses in 2015, up from 774 in 2014. The bulk of the arrests were in France, followed by Spain and the U.K.

Jihadist terror attacks in Europe in 2015 were plotted by people who had returned from fighting in Syria, used local recruits to carry out the attacks and likely received direction from Islamic State leaders, according to the Europol report.

Islamic State, according to the report, often threatens countries participating in the U.S.-led coalition against the militant group and has made specific threats to Belgium, France, Italy and the U.K. in its propaganda.

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