Candles and flowers left at the entrance of the Notre-Dame basilica in Nice, France, last month after an attack linked to Islamist extremists. British police say the coronavirus pandemic and the subsequent social isolation have contributed to a “perfect storm” that is making more young people vulnerable to radicalization. Neil Basu, the assistant commissioner of the Metropolitan Police, said that police had already seen a sharp increase in extremist material online and, with young people spending more time online in isolation without the protective influence of schools and support networks, they were increasingly at risk. “In my opinion that is a perfect storm, one which we cannot predict and that we might be feeling the effects of for many years to come,” Mr. Basu said in a statement released by the police on Wednesday. The comments come after a series of attacks in France and a shooting this month in Vienna, all linked to Islamist extremists. Britain’s terror threat level was raised to its highest — severe — in the wake of the violence, meaning authorities suspect an attack is likely, though without a specified threat.