Two people were arrested near Vienna on Tuesday, according to the APA news agency, as police hunted accomplices of a gunman or gunmen who killed four people in what the government called a terrorist attack in the Austrian capital.
In an early morning televised news conference, Interior Minister Karl Nehammer repeated calls for the public to stay off the streets.
Nehammer said police had shot dead one attacker on Monday night, a man wearing an explosives belt that turned out to be fake. Authorities identified him as an Islamic State sympathizer. Authorities and witnesses were not immediately able to rule out the possibility there had been other gunmen.
Police confirmed on Tuesday that three civilians - two men and a woman - had been killed, and at least 15 others wounded, including a police officer. The national broadcaster ORF later said a fourth civilian, a woman, had died.
APA quoted police as saying two arrests had been made in the nearby town of St Poelten. The Kurier news site reported that heavily armed police had searched two properties.
Witnesses had described crowds being fired on in bars with automatic rifles on Monday night, as many people took advantage of a last evening out before the start of a nationwide coronavirus curfew.
Six locations were attacked in central Vienna, starting outside the main synagogue, which was closed.
A witness, Vienna rabbi Schlomo Hofmeister, said he could not be certain whether there was more than one shooter.
"I saw one person. Later, I saw videos and I'm not sure it was the same one. I find it very difficult to identify someone in a fraction of a second," Hofmeister told Reuters television.
"At that moment I also did not pay attention to what he looks like. I was trying to understand what's happening. I then moved away from the window and called the police."
Nehammer, the interior minister, said: "We experienced an attack yesterday evening by at least one Islamist terrorist, a situation that we have not had to live through in Austria for decades."
Seven of the injured were in a life-threatening condition, the APA news agency said.
A police spokesman said at least 1,000 officers were involved in the search.
The government announced three days of national mourning, and a minute's silence at noon. (1100 GMT)
The editor of Vienna's Falter newspaper, Florian Klenk, tweeted that the dead assailant was a 20-year-old born in Austria to ethnic Albanian parents from North Macedonia, and known to domestic intelligence agencies as one of 90 Austrian militants who aimed to travel to Syria. Klenk did not give a source for his information.
Nehammer said video material had been seized from the home of the known assailant, and police were investigating his potential connections.
APA reported that multiple homes had been searched and arrests made, citing the Interior Ministry. A Ministry spokesman was not immediately available to comment.
Vienna's police chief declined to provide further details on the known attacker's identity, saying it might endanger the investigation.
Police sealed off much of the historic centre of the city overnight, urging the public to shelter where they were. Many sought refuge in bars and hotels, while public transport in the area was shut down.
Oskar Deutsch, head of Vienna's Jewish community, which has offices adjoining the synagogue on a narrow cobbled street dotted with bars, tweeted that it was not clear whether the synagogue or offices had been a target.
Videos circulated on social media of a gunman running down a cobbled street shooting and shouting. One showed a man gunning down a person outside what appeared to be a bar on the street where the synagogue is located.
Vienna had been spared the kind of deadly militant attacks that have struck Paris, London, Berlin and Brussels, among others, in recent years.
Condolences poured in from leaders around the world. US President Donald Trump tweeted that "our prayers are with the people of Vienna after yet another vile act of terrorism in Europe".
"These evil attacks against innocent people must stop. The US stands with Austria, France, and all of Europe in the fight against terrorists."
Democratic presidential candidate Joe Biden condemned what he called a "horrific terrorist attack", adding: "We must all stand united against hate and violence."