One person is dead and four others injured after a knife attack in a supermarket in the German city of Hamburg. The motive for the attack is not yet known. A German newspaper showed a photo of the alleged attacker in the back of a police car. Passersby are reported to have tackled the suspect after he fled the scene and he suffered some injuries. Police say they believe the assailant acted alone. Germany has been on high alert for terrorist attacks for months.