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 authorities said they were struggling to understand the motives for the attack with a car in the eastern city of Magdeburg, in which a 9-year-old boy was among the five victims.