The suspect in a knife attack that killed a 14-year-old boy and injured five others in Austria on Saturday was an “Islamic extremist attacker” who had “connections to IS”, investigators have said.

Witnesses said the 23-year-old Syrian asylum seeker shouted “Allahu Akbar” after the attack, in which he used a flick-knife with a ten-inch blade against bypassers in the town of Villach.

The suspect “radicalised himself on the internet in a short time”, interior minister Gerhard Karner told journalists at a press conference, while police said the man, who is being charged with murder and attempted murder, had recorded himself swearing an oath of allegiance to IS.

A photo taken after the attack showed the suspect grinning and showing a single raised finger, a symbol of the so-called Islamic State terror group, which has launched multiple attacks in Germany and Austria in recent years.

Police found “clear indications of a radical Islamic ideology” such as an IS-flag on his wall. No weapons were found, and accomplices will be investigated.