An unidentified assailant on Friday attacked a crowd celebrating the “Festival of Diversity,” which is part of a series of events marking the city’s 650th anniversary.
The attacker stabbed three people to death and injured eight others, four of whom are in critical condition.
ISIS propaganda outlet Amaq announced that the perpetrator of the attack was one of their fighters and claimed that he carried out the stabbing in retaliation for “Muslims in Palestine and everywhere.”
There is a large police search for the perpetrator. A 15-year-old has been arrested, but police declined to give any further information.
Markus Caspers, Dusseldorf prosecutor, told journalists on Saturday that witnesses reported seeing a group of teenagers discussing the attack with a man, suspected to be the assailant.
"We have not been able to identify a motive for now, but in view of all of the circumstances, we are working under the assumption that the initial suspicion of a terrorist motive cannot be excluded," Caspers said.
German Interior Minister Nancy Faeser said that "security authorities are doing everything they can to catch the perpetrator" of the "horrific act."
Chancellor Olaf Scholz said that the assailant "must be caught quickly and punished."
The attack began as thousands of people had gathered in front of a stage on the festival's first night.
Znar Shino contributed from Solingen, Germany.
