Police have carried out a series of raids and made 14 arrests in the hours since a gunman murdered four people in the heart of Vienna.
Security has been tight in the Austrian capital as police launched a manhunt for further attackers.
But authorities believe the gunman shot dead by police may have acted alone.
Interior Minister Karl Nehammer has not ruled out the possibility of a second attacker. Some witnesses spoke of seeing more than one gunman and police are still assessing some 20,000 mobile phone videos of what happened.
“Over 50% have already been watched but the assessment is not over. At the same time the videos already evaluated do not indicate that there was a second perpetrator,” the minister told reporters.
The attacks began at 20:00 local time (19:00 GMT) on Monday night, opposite the main synagogue in a pedestrian alley called Seitenstettengasse, and continued until the gunman was shot dead at 20:09.
Police identified a total of six crime scenes in the area, as witnesses described seeing attackers opening fire on people outside bars and then chasing them inside.
Austrian Chancellor Sebastian Kurz said the four who died were an elderly woman, an elderly man, a young male passer-by and a waitress. Another 22 people were wounded.
One of the women who died was German, Germany’s Foreign Minister Heiko Maas tweeted.
It was clearly an attack driven by “hatred of our way of life, our democracy”, Chancellor Kurz said.
A special service to commemorate the victims will be held at Vienna’s St Stephen’s Cathedral from 18:00 (17:00 GMT), attended by Christian, Jewish and Muslim leaders and senior politicians including Chancellor Kurz.
Three days of national mourning have begun, with flags flying at half-mast. Children – those in Vienna kept at home for safety on Tuesday – will observe a minute’s silence at school on Wednesday.
It has also emerged that a police officer who was shot and wounded by the gunman was taken to safety by two men of Turkish descent.
“I took his legs, he took his shoulders, the police were there too of course and helped, and we quickly got him to the ambulance,” Mikail Özen, a personal trainer and mixed martial arts fighter, told journalists.
Mr Nehammer praised them, saying “more than ever: if people think that they can divide our society with violence, terror and fear it’s important to stand together”.
He said 14 people linked to the gunman had been detained. Most of the arrests were close to where he lived but two suspects were also arrested in St Pölten, to the west of Vienna, and one arrested in the city of Linz.
What we know about the gunman
Earlier, Mr Nehammer described the dead gunman as an “Islamist terrorist”, who had been jailed for 22 months in April 2019 for trying to travel to Syria to join Islamic State (IS) jihadists.
The 20-year-old had been released early last December under more lenient terms for young adults. He had managed to convince the authorities that he no longer held extremist Islamist views, Mr Nehammer said.
Originally from North Macedonia, he had both Austrian and Macedonian citizenship.
Police have searched his home and seized video material. As well as being heavily armed, he was wearing a fake explosive belt, police said.
Mr Nehammer said the gunman opened fire with a sawn-off Kalashnikov assault rifle, but was also armed with a machete and a pistol.
The attack came hours before Austria imposed new national restrictions to try to stem rising cases of coronavirus. Many people were enjoying having a drink or eating out before a midnight curfew.
Elsewhere in Europe, France has suffered a spate of Islamist militant attacks recently, involving “lone wolf” jihadists.
Last month French history teacher Samuel Paty was beheaded outside a school in a suburb of Paris by a Chechen youth, who was then shot dead by police.
As the French government launched new measures to tackle militant Islam, a Tunisian man fatally stabbed three people in a cathedral in Nice.
The worst IS attack in Europe in recent years was in November 2015, when gunmen killed 130 people in Paris.