Four women and a 9-year-old boy were killed and 200 people were injured when a man drove into a Christmas market teeming with holiday shoppers in the German city of Magdeburg — an attack that has left Germans mourning the victims and with a shaken sense of security.

The latest deadly assault on a beloved German tradition illustrates the challenge of holiday safety. For years, Germany has been strengthening security measures around Christmas markets in reaction to an attack in 2016 in which a man killed 12 people by driving a truck into a market in central Berlin.