Special police units on Saturday joined the search for an unknown man who carried out a stabbing attack at a crowded festival in the western German city of Solingen, killing three people and wounding at least eight others, four of them seriously.

A 15-year-old boy was arrested early on Saturday.

Police said he was suspected of knowing about the planned attack and not informing authorities, but he was not the attacker.

Markus Caspers, from the counterterrorism section of the public prosecutor’s office, told a news conference on Saturday that authorities have not found the perpetrator.

“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, Mr Caspers said, though he did not offer further details.

The three people who died were two men aged 67 and 56 and a woman aged 56, authorities said.

Police said the attacker appeared to have deliberately aimed for his victims’ throats.

Thorsten Fleiss from the German police, who was the chief of operations on Friday, said police are conducting various searches and investigations in the entire state of North Rhine Westphalia that will continue throughout the day.

He said that it is a “big challenge” to bring together available evidence and testimony from witnesses in order to come up with a overall picture.

Mr Fleiss also said police have found several knives but added that he was unable to confirm whether any of them had been used as a weapon by the perpetrator during the attack.

Police told people to stay vigilant even as wellwishers 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.

German interior minister Nancy Faeser paid a visit to Solingen on Saturday evening.

She said that the government would do everything possible to support the city and the people of Solingen.

“We will not allow that such an attack divides our society,” she said, appearing alongside minister-president of the German State of North Rhine Westphalia Hendrik Wust and state minister for internal affairs Herbert Reul.

Mr Wust described the attack as “an act of terror against the security and freedom of this country”.

But Ms Faeser, the country’s top security official, has not classified it as a “terror attack”.

Mr Reul announced that the planned visit of the interior minister to the crime scene would not take place because of the ongoing police operation in the affected areas of the city.

He pleaded with the public to “give time to the police” so that they can do their work.

He also said that the police presence would be increased at larger events, especially because the perpetrator has not been caught yet.

People alerted police shortly after 9.30pm on Friday to an unknown attacker having wounded several people with a knife on a central square, the Fronhof.

Police said they believe the stabbings were carried out by a lone attacker and gave no information about the identities of the victims.

“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, said, speaking to reporters on Saturday near the scene of the attack.

The Festival of Diversity, marking the city’s 650th anniversary, began on Friday and was supposed to run until 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 after the attack, the stage lights were still on as police and forensic investigators looked for clues in the cordoned-off square.

One of the festival organisers, Philipp Muller, appeared on stage on Friday and asked festivalgoers to “go calmly, please keep your eyes open, because unfortunately the perpetrator hasn’t been caught”.

Solingen has about 160,000 residents and is near the bigger cities of Cologne and Dusseldorf.

The rest of the festival was cancelled.

German Chancellor Olaf Scholz said on Saturday that the perpetrator of the attack must be caught quickly and 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,” Mr Scholz said on X, formerly Twitter.

German President Frank-Walter Steinmeier also spoke to the mayor of Solingen on Saturday.

“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,” Mr Steinmeier said in a statement on Saturday.

“The perpetrator needs to be brought to justice. Let’s stand together — against hatred and violence.”

There has been concern about increased knife violence in Germany, and interior minister Nancy Faeser recently proposed toughening weapons laws to allow only knives with a blade measuring up to six centimetres to be carried in public, rather than the 12 centimetres that is currently allowed.