The gunmen who mowed people down in cafes in central Paris and at a packed theater in the French capital on Friday were acting in a city on high alert since terrorists attacked satirical newspaper Charlie Hebdo and a Jewish supermarket in January.

That did nothing to stop them.

According to eyewitnesses, the men with machine guns walked down streets crowded with Friday revelers with aplomb to carry out the worst terror attack in over a decade in a Western country. Their action shows the challenge of providing foolproof security in a city of 2.2 million people with a bar or a restaurant dotting practically every corner, said Louis Caprioli, the ex-head of DST, France’s former anti-terrorism unit, and now an adviser to Paris-based security consultants Groupe GEOS.

… “This kind of attack is an intelligence failure,” said Rohan Gunaratna, head of the International Centre for Political Violence and Terrorism Research in Singapore. France was “aware of the threat but did not anticipate an attack of this scale.”