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The Islamic State militant group on Saturday claimed responsibility for a knife attack that killed three people and wounded eight more at a crowded festival marking this city's 650th anniversary.
The extremist group said on its news site that the attacker targeted Christians and that as a "soldier of the Islamic State" he carried out the assaults on Friday night "to avenge Muslims in Palestine and everywhere."
The IS claim couldn't immediately be verified. It provided no evidence for its assertions.
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Police later detained a suspect, the internal affairs minister of North Rhein Westphalia state said early on Sunday.
"We have been following a hot lead all day," Herbert Reul told Tagesschau, the news program of the German public television network ARD.
"The person we have been searching for all day has been detained a short while ago."
He was being questioned, Reul said.
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Reul said police not only had "clues" but also collected "pieces of evidence."
Officials earlier said a 15-year-old boy was arrested early on Saturday on suspicion he knew about the planned attack and failed to inform authorities, but he was not the attacker.
Two female witnesses told police they overheard the boy and an unknown person before the attack speaking about intentions that corresponded to the bloodshed, officials said.
Before the Reul announcement, Markus Caspers, senior public prosecutor from the counterterrorism section of the public prosecutors office, said at a news conference on Saturday that authorities could not yet speak on the attacker's motivation.
"So far we have not been able to identify a motive, but looking at the overall circumstances, we cannot rule out" the possibility of terrorism, Caspers said, though he did not offer further details.
The three people who died were two men aged 67 and 56 and a 56-year-old woman, authorities said.
Police said the attacker appeared to have deliberately aimed for his victims' throats.
"We are seeing the first signs of a new wave of terrorist attacks," said Peter Neumann, a professor of security studies at King's College in London.
"IS is trying to capitalize on the huge mobilisation resulting from Hamas' terror offensive on 7 October 2023, even though strictly speaking it had nothing to do with it," he said.
"The kind of attack we saw in Solingen is exactly the kind of attack that (IS) is trying to inspire. It's calling on people over the internet to attack 'unbelievers' using simple methods like cars and knives.
"That way, it is trying to create an impression that (the Islamic State group) is everywhere and could strike anytime," Neumann told The Associated Press.
Thorsten Fleiss, who headed police operations on Friday night, said officers were conducting searches and investigations in the entire state of North Rhine Westphalia.
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He said police had found several knives, but added he was unable to confirm whether any of them were used during the attack.
Police warned people to stay vigilant even as well wishers started to leave flowers at the scene.
Authorities established an online portal where witnesses could upload footage and any other information relevant to the attack.
Churches in Solingen opened their doors to offer a space for prayer and emergency pastoral care.
German Interior Minister Nancy Faeser visited the city on Saturday evening and said the government would do everything possible to support the people of Solingen.
"We will not allow that such an awful attack divides our society," she said, appearing alongside state Minister-President Hendrik Wüst and Reul.
Wüst described the attack as "an act of terror against the security and freedom of this country." But Faeser, the country's top security official, had not classified it as a "terror attack."
People alerted police shortly after 9.30 pm on Friday that a man had assaulted several people with a knife on the city's central square, the Fronhof.
Solingen, a city of about 160,000 residents near the bigger cities of Cologne and Duesseldorf, was holding a "Festival of Diversity" to mark its 650th anniversary.
It began on Friday and was supposed to run through to Sunday, with several stages in central streets offering attractions such as live music, cabaret and acrobatics.
The attack took place in the crowd in front of one stage. Hours later, the stage lights were still on as police and forensic investigators looked for clues in the cordoned-off square, but the rest of the festival was cancelled.
"Last night our hearts were torn apart. We in Solingen are full of horror and grief. What happened yesterday in our city has hardly let any of us sleep," the mayor of Solingen, Tim Kurzbach, told reporters on Saturday.
German Chancellor Olaf Scholz said the perpetrator must be punished with the full force of the law.
"The attack in Solingen is a terrible event that has shocked me greatly. An attacker has brutally killed several people. I have just spoken to Solingen's mayor, Tim Kurzbach.
"We mourn the victims and stand by their families," Scholz said Saturday on the social media platform X.
German President Frank-Walter Steinmeier also spoke to the mayor on Saturday morning.
"The heinous act in Solingen shocks me and our country. We mourn those killed and worry about those injured and I wish them strength and a speedy recovery from all my heart," Steinmeier said in a statement.
A decade after the Islamic State militant group declared its caliphate in large parts of Iraq and Syria, the extremists no longer control any land, have lost many prominent leaders and are mostly out of the world news headlines.
Still, the group continues to recruit members and claim responsibility for deadly attacks around the world, including lethal operations in Iran and Russia earlier this year that killed dozens of people.
Its sleeper cells in Syria and Iraq still carry out attacks on government forces in both countries as well as US-backed Syrian fighters.
