After an assailant driving an S.U.V. killed five people, calls for solidarity have quickly given way to criticism by rival lawmakers, ahead of snap elections set for February.

The German government pledged Sunday to fully investigate whether there were security lapses before the Christmas market car-ramming attack that killed five people and injured over 200.

The authorities said they were struggling to understand the motives behind ramming a car into a Christmas market in the eastern city of Magdeburg, which left a 9-year-old boy among the five dead.

The latest deadly assault on a beloved German tradition illustrates the challenge of holiday safety.