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

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.

At least 12 people were injured in a knife attack in Hamburg station on Friday evening, Germany's Bild newspaper reported, and local police confirmed they had arrested the suspected assailant. Three victims are in critical condition,