Austrian authorities say the stabbing of six passersby that left a 14-year-old boy dead was carried out by a man with possible connections to the Islamic State group who appeared to have acted alone and motivated by radical Islamic ideology.

The suspect, a 23-year-old Syrian, was arrested after the attack in the southern city of Villach on Saturday. Five others were wounded, two of them seriously.

“This is an Islamist attack with an IS connection by an attacker who radicalized himself within a very short time via the internet online,” Interior Minister Gerhard Karner told reporters in Villach Sunday.

As the focus shifted to migration and asylum-seekers, Mr. Karner said that it will ultimately be necessary to “carry out a mass screening without cause because this assassin was not conspicuous.”

He did not elaborate, but Austria’s right-wing leader Herbert Kickl, whose party won a national election four months ago, called for “a rigorous crackdown on asylum” in the wake of the attack.

The mayor of Villach, Günther Albel, said the attack was a “stab in the heart of the city.”

According to state police director Michaela Kohlweiß, police searched the suspect’s apartment with sniffer dogs and found IS flags on the walls.

No weapons or dangerous objects were found, she added but police seized mobile telephones.

Police were investigating whether the suspect had any accomplices.

“The current picture is that of a lone perpetrator,” Ms. Kohlweiß said. It is assumed that the suspect, who was not known to police, was quickly radicalized online, she said. Police presence will be increased in the streets of Villach and at events in the weeks ahead, Ms. Kohlweiß added.

The attacker used a folding knife, Ms. Kohlweiß said. Those wounded include two 15-year-old boys, a 28-year-old man, a 32-year-old man, and a 36-year-old man. Four of them are receiving hospital treatment and one is being treated for minor injuries.

This marks the second deadly jihadi attack in Austria in recent years. In November 2020, a man who had previously attempted to join the Islamic State group carried out a rampage in Vienna, armed with an automatic rifle and a fake explosive vest, killing four people before being fatally shot by police.

Residents began placing candles at the site of the attack in the city of about 60,000 inhabitants. A group of young people, who knew the boy who died in the attack, gathered at the crime scene on Sunday morning to mourn and tearfully light candles, local media reported.

Austria’s President Alexander Van der Bellen called the attack “horrific.”

“No words can undo the suffering, the horror, the fear. My thoughts are with the family of the deceased victim and the injured,” he posted on X.

The Free Syrian Community of Austria issued a statement on Facebook distancing itself from the attack and expressing its deepest condolences to the victims’ families. “We all had to flee Syria, our home country, because we were no longer safe there — no one left their country voluntarily. We are grateful to have found asylum and protection in Austria,” the association said.

Villach, a popular tourist destination near the borders of Italy and Slovenia, is known for its laid-back atmosphere, which blends Mediterranean and Alpine traditions. The city hosts annual carnival processions in March and an event on Saturday was canceled in the wake of the attack.

The Austrian Ministry of Interior activated a platform for witnesses to upload videos or photos related to the attack. Local authorities said a crisis response team will be available to support pupils when schools open on Monday.

Mr. Kickl wrote on X Saturday that he is “appalled by the horrific act in Villach.”

“At the same time, I am angry — angry at those politicians who have allowed stabbings, rapes, gang wars and other capital crimes to become the order of the day in Austria. This is a first-class failure of the system, for which a young man in Villach has now had to pay with his life,” Mr. Kickl said.

“From Austria to the EU — the wrong rules are in force everywhere. Nobody is allowed to challenge them, everything is declared sacrosanct,” he said, adding that his party had outlined what he viewed as necessary changes to immigration laws in its election platform.

Conservative party leader Christian Stocker said on X that the attacker “must be brought to justice and be punished with the full force of the law.”

“We all want to live in a safe Austria, adding that this means political measures need to be taken to avoid such acts of horror in the future,” he said.

The leader of the Social Democrats, Andreas Babler, said, “Crimes like this one simply should not happen in our society.”