The Islamic State group claimed responsibility on Saturday for a knife attack in the western German city of Solingen that killed three people and injured eight others.

The militant group said in a statement on its Telegram account that the attack was carried out by one of its members “in revenge for Muslims in Palestine and everywhere”.

It did not immediately provide any evidence for its assertion and it was not clear how close any relationship between the attacker and Islamic State was.

Earlier, police in Germany had said they did not rule out a “terrorist motive” after the mass stabbing at a festival on Friday night.

At a press briefing, police said they had detained a 15-year-old at his parents’ home in the early hours of Saturday in connection with the attack in Solingen, west Germany. But they added that he is not believed to be the prime suspect, who is still unknown.

Public prosecutor Markus Caspers said of the 15-year-old: “He is at this point only suspected of failing to report a crime.” He added that the suspect was alleged to have spoken with the perpetrator “shortly before the crime”.

He said that a “terrorist motive” could not be excluded, partly because the assailant did not appear to know his victims.

A woman, 56, and two men, 56 and 67, all from the region, were killed in the attack on Friday night, authorities said.

Police found at least one weapon that may have been used in the assault and are analysing it for DNA traces. They said they had had no indication in the run-up to the festival that there was a security threat.

People began leaving flower bouquets and candles in tribute to the victims at the site of the attack in the centre of Solingen.

Authorities set up a website for people to send in footage or information about the attack as well as a telephone hotline, and urged witnesses not to post relevant videos directly to social media.

The assailant, who is still at large, used a knife to attack people apparently at random in a crowd of thousands gathered for a festival at the central square in Solingen on Friday night. The frenzied assault, which happened at a festival of diversity during celebrations to mark the city’s 650th anniversary, lasted only minutes, witnesses said.

Germany’s chancellor, Olaf Scholz, who has been under pressure to fight a rise in knife violence in cities, said on Saturday that he was “shocked” by the “terrible event” and stood with the terrorised city in mourning the victims.

“I wish the injured a speedy recovery,” he said in a post on X. “The perpetrator must be caught quickly and punished to the full extent of the law.”

The authorities have deployed a “large contingent”, including helicopters, to search for the male assailant who fled the scene, and established road checkpoints. A spokesperson said it remained unclear which direction he had fled in and what means of transport he used.

“Both victims and witnesses are currently being questioned. The police are currently searching for the perpetrator with a large team,” the spokesperson said, urging the public to be vigilant and cautious.

Herbert Reul, the interior minister of North Rhine-Westphalia state – Germany’s most populous region, warned against speculation about the perpetrator’s background.

“You don’t want to believe what you see here at the crime scene. It’s upsetting,” Reul said.

“Out of nowhere, someone stabs people indiscriminately. We can’t say anything about the person or the motive yet,” he said, acknowledging that the police had very few leads.

The mass stabbing happened at a festival that was supposed to run through to Sunday, drawing up to 25,000 people each day with a programme including live bands, cabaret acts, acrobats and entertainment for children. The rest of the festival has now been cancelled, as were weekend festivities in nearby towns.

A police spokesperson said emergency services had received several calls at about 9.40pm with witnesses reporting that “an unknown person armed with a knife wounded several people at random”.

A large crowd had gathered around a stage with live music on the Fronhof market square in the city centre. Most of those injured are believed to have been attacked directly in front of the stage, the daily newspaper Bild reported, adding that the man appeared to target the throats of his victims.

The German DJ Topic, who is from Solingen, said in a post on Instagram he was performing on the stage when security personnel approached him and informed him of the attack.

He was asked to continue his set “to avoid causing a mass panic”, he said. “So I kept playing even though it was incredibly hard.” He said he was told to stop 10-15 minutes later, and “since the attacker was still on the run, we hid in a nearby store while police helicopters circled above us,” he wrote.

“I still can’t believe it … this was supposed to be a free festival for everyone. Really close friends of mine were there with their small kids,” he said in a video recorded in his childhood bedroom. “What’s happening to this world … my thoughts are with all the victims.”

Sascha Mosig, who was on his first night on the job for a security firm at the festival, said he suddenly saw a group of people running in his direction, some of them covered in blood. One screamed, “Knife.”

The 37-year-old told weekly newspaper Die Zeit that he went to the main square to help and saw lifeless bodies on the ground and people in shock.

“Blood was everywhere,” he said. “You know these images from war. This was one.”

Another witness, Lars Breitzke, told the local newspaper Solinger Tageblatt that he was a few metres from the attack, not far from the stage, and “understood from the expression on the singer’s face that something was wrong”.

“And then, a metre away from me, a person fell,” said Breitzke, who at first thought it was someone who was drunk. But when he turned around, he saw other people lying on the ground and several pools of blood, he added.

The authorities called on people to maintain calm as they left the city centre and witnesses said festivalgoers complied, avoiding a crush.

On Saturday morning, armed police were guarding a security perimeter, with witnesses describing a “ghostly” atmosphere in the normally bustling shopping district.

Solingen has about 160,000 inhabitants and is located near the bigger cities of Cologne and Düsseldorf.

The federal health minister, Karl Lauterbach, said he hoped “rescue teams can save the wounded who are still alive and that police can catch the cowardly and pathetic perpetrator”. The foreign minister, Annalena Baerbock, said the “treacherous attack … shocked me deeply”.

The federal interior minister, Nancy Faeser, said security authorities were “doing everything they can to catch the perpetrator and determine the background to the attack”.

Germany has experienced a series of knife attacks over the past 12 months, with Faeser pledging earlier this month to crack down on knife crime with a reformed weapons law.

In May, German police shot and wounded a man who injured six people in a knife attack at a rightwing demonstration in the south-western city of Mannheim. Among the victims was a 29-year-old policeman who intervened and was fatally stabbed.