Eighteen people were injured – four of whom remain in critical condition – in a knife attack Friday in the German city of Hamburg, authorities said.

Hamburg police said on Saturday that four of the victims who had sustained life-threatening injuries were in a stable condition. Officers arrested a 39-year-old German woman at the scene of the attack, which took place at about 18:00 local time (16:00 GMT) on Friday.

Four of the victims are in critical condition. The police say they believe a 39-year-old woman carried out the attack, for which no motive has yet been given.