The shocked nation began three days of mourning after what Austrian Chancellor Sebastian Kurz described as a “repulsive terror attack”.
One of the suspected killers, who was shot dead by police after Monday's attack, was a 20-year-old sympathiser of the terror group Isis with dual Austrian and Macedonian nationality, Interior Minister Karl Nehammer said.
The attacker managed to “fool” de-radicalisation efforts after his conviction for terrorist offences, Austria's interior minister said Tuesday.
“The perpetrator managed to fool the de-radicalisation programme of the justice system, to fool the people in it, and to get an early release through this,” Interior Minister Karl Nehammer told a press conference.
He had been convicted of a terror offence in April last year for trying to travel to Syria, Nehammer told the APA news agency.
According to Nehammer, police are searching for more possible assailants who may still be at large, and several neighbouring countries have stepped up border checks.
“It's difficult for us at the moment to define whether the attack was carried out by one perpetrator or more than one,” said Gerhard Puerstl of the Vienna police.
The shooting erupted just hours before Austria was to re-impose a coronavirus lockdown, with people out in bars and restaurants enjoying a final night out.
Austria had until now been spared the sort of major attacks that have hit other European countries, but Monday's shooting followed a spate of Islamist attacks in France and it triggered an outpouring of solidarity from world leaders.
Across the country, flags have been lowered to half mast on public buildings and people observed a minute of silence at noon (1100 GMT) as church bells rang out.
'Sounded like firecrackers'
Two men and two woman were killed in the attack, an interior ministry spokesman said, while 14 more have been injured, six seriously.
Police also said an officer had been hurt. The first shots were heard at around 8 pm (1900 GMT) in the heart of the city near a synagogue and the world famous opera house.
“It sounded like firecrackers, then we realised it was shots,” said one witness quoted by public broadcaster ORF.
A gunman “shot wildly with an automatic weapon” before police arrived and opened fire, the witness added. Another spoke of at least 50 shots being fired.
Nehammer told a press conference earlier Tuesday that police had used explosives to blast their way into the apartment of the dead suspect who had been “heavily armed”.
“All the signs make it clear it's a radicalised person and a person who feels closely connected to IS.”
Nehammer urged Vienna residents to remain in their homes and said children would not be expected to go to school on Tuesday.
'Never be intimidated'
Speaking to ORF, Kurz said the attackers “were very well equipped with automatic weapons” and had “prepared professionally”.
He also tweeted: “Our police will act decisively against the perpetrators of this repulsive terror attack. “We will never be intimidated by terrorism and we will fight this attack with all means.”
Kurz said that while police were concentrating on an anti-terror operation, the army would take over the security of major buildings in Vienna.
The president of Vienna's Jewish community Oskar Deutsch said shots had been fired “in the immediate vicinity” of the Stadttempel synagogue, but added that it was currently unknown whether the temple — closed at the time — had been the target.
At the busy bars and restaurants, people were told to remain indoors as the attack unfolded.
“At the beginning, I thought to myself that maybe we were making an American film or that they had drunk too much,” said waiter Jimmy Eroglu, 42.
But then he heard shots. “The police came in and said, 'you all have to stay inside because there's a probably a dead man there'.” Robert Schneider, who lives in central Vienna, went out and found two lasers trained on his chest.
“Hands up, take off your jacket,” officers shouted at him, the 39-year-old told AFP. “We had seen nothing, heard nothing. We are in shock.”
Germany joined the Czech Republic in stepping up checks at their borders with Austria as Chancellor Angela Merkel said: “The fight against these assassins and those who instigate them is our common struggle.”
President Emmanuel Macron said the French shared the “shock and sorrow” of the Austrian people.
“After France, it is a friendly nation that has been attacked. This is our Europe. Our enemies must know who they're dealing with. We will concede nothing,” he tweeted in both French and German.
France is still reeling from the killing of three people at a church in the Riviera city of Nice last Thursday and the beheading of a schoolteacher by a suspected Islamist outside Paris on October 16.
Leaders of other nations also voiced support for Austria.
“These evil attacks against innocent people must stop. The US stands with Austria, France, and all of Europe in the fight against terrorists, including radical Islamic terrorists,” US President Donald Trump said.