The 16-hour siege in Sydney’s central business district that ended with the death of a gunman and two hostages is the latest in a recent series of lone-wolf incidents that are posing new challenges for security agencies.

Before the Lindt Chocolat Cafe was stormed by police just after 2 a.m. today, television images showed captives holding up a black flag with white Arabic lettering. Islamic State has urged Muslims worldwide to retaliate for allied attacks on the extremist group in Iraq and Syria. The prospect that individuals are responding to that call is making it harder for anti-terror agencies to act.

…The speech by Islamic State spokesman Abu Muhammad al-Adnani entitled “Indeed Your Lord is Ever Watchful” in September had “served not only to rally IS fighters in Iraq and Syria against the U.S.-led coalition, but to mobilize jihadists in Western countries,” said Rohan Gunaratna, head of the International Centre for Political Violence and Terrorism Research at Nanyang Technological University in Singapore.