Austrian investigators were on Tuesday (November 3rd) piecing together the Monday evening rampage through central Vienna by a lone gunman that has since been claimed by the "Islamic State of Iraq and Syria" (ISIS).

Four people were killed when Kujtim Fejzulai, described as a 20-year-old ISIS sympathiser who had spent time in prison, opened fire with a Kalashnikov in a busy area of the Austrian capital.

Police said he was also wearing a fake explosive belt.

During a raid on Fejzulai's home, investigators found incriminating evidence on his computer, including a photograph posted on Facebook showing him posing with the automatic weapon and a machete used during the attack.

The photo caption claimed he was "serving the sultanate" and other typical ISIS messages, Austrian Interior Minister Karl Nehammer said.

ISIS on Tuesday claimed it was responsible for the carnage, according to its propaganda agency, Amaq.

Amaq also published a short video in which the armed perpetrator filmed himself pledging allegiance to the head of the extremist group.

Police shot Fejzulai dead on Monday and later swooped in on 18 different addresses and made 14 arrests as they looked for possible accomplices.

'Repulsive' attack

Austrian Chancellor Sebastian Kurz condemned the shooting as a "repulsive terror attack", which he said killed a waitress, a young passer-by and an older man and woman.

His government will face questions about how an individual known to security forces had been able to buy weapons and cause havoc on the streets of the usually peaceful capital, often listed as having the world's highest quality of life.

The investigation is spanning several countries, with Switzerland making two arrests and Macedonia, where Fejzulai has family roots, co-operating with the Austrian authorities.

Two young Swiss men who were arrested near Zurich on Tuesday in connection with the deadly rampage allegedly knew the gunman, the justice ministry said.

"They know each other," justice ministry spokesman Philipp Schwander said, adding that investigators were probing "how they were in contact".

Nehammer said Fejzulai had been convicted and jailed in April last year for trying to travel to Syria and join ISIS.

The dual Austrian and Macedonian national had then been admitted to a government-funded de-radicalisation programme and had managed to secure an early release in December from a 22-month prison sentence.

Austria's top security chief Franz Ruf told local media that at his last session of the de-radicalisation programme in late October, Fejzulai had condemned the recent extremist attacks in France.

There had been a previous attempt on the part of Vienna authorities to strip Fejzulai of his Austrian citizenship, but this was not successful because "there was not enough evidence about his activities", Nehammer said.

'Shock and sorrow'

The bloodshed triggered an outpouring of solidarity from world leaders.

Across the country, flags were lowered to half mast on public buildings and people observed a minute of silence at noon as church bells rang out.

Kurz, President Alexander Van der Bellen and other officials took part in a wreath-laying ceremony in honour of the victims.

Twenty-three wounded people were in hospital Tuesday, with seven in a critical condition, Vienna's hospital association said.

Police said an officer was among those hurt.

Germany joined the Czech Republic in stepping up checks at their borders in order to stop possible accomplices.

"The fight against these assassins and those who instigate them is our common struggle," said Chancellor Angela Merkel.