Austria's top security official says the attacker killed in Vienna was a 20-year-old dual citizen of Austria and North Macedonia who had a previous terrorism conviction.
Interior Minister Karl Nehammer told Austria's APA news agency that 15 house searches have taken place and several people have been arrested.
Five people have died, including an assailant, and 17 others were wounded in a shooting in the heart of Vienna hours before a coronavirus lockdown started, Austrian authorities said Tuesday.
Interior Minister Karl Nehammer said two men and two women have died from their injuries in the attack Monday evening. A suspected attacker, who was carrying an assault rifle and a fake suicide vest, was also shot and killed by police.
"We experienced an attack last night by at least one Islamist terrorist," Nehammer told reporters. He declined to elaborate, citing the ongoing investigation.
Vienna's hospital service said seven people were in life-threatening condition Tuesday after the attack, the Austrian news agency APA reported. In total, 17 people were being treated in hospitals, with gunshot wounds but also cuts.
Nehammer said that initial investigations indicate the suspect who was killed had sympathized with the Islamic State group. Police searched his apartment and other premises as well, APA reported.
Nehammer said officials believe there was more than one attacker, adding that 1,000 security personnel had been deployed for the manhunt while neighboring countries had offered assistance.
"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," Nehammer said.
"Austria for more than 75 years has been a strong democracy, a mature democracy, a country whose identity is marked by values and basic rights, with freedom of expression, rule of law, but also tolerance in human coexistence," he said. "Yesterday's attack is an attack on just these values."
Gunmen attacked six locations in central Vienna on Monday evening, starting outside the main synagogue. Witnesses described the men firing into crowds in bars with automatic rifles, as many people took advantage of the last evening before a nationwide curfew was introduced because of COVID-19.
Witnesses described the men firing into crowds in bars with automatic rifles, as many people took advantage of the last evening before a nationwide curfew was introduced because of COVID-19. Police shot and killed one assailant.
Police sealed off much of the historic center of Vienna, urging the public to shelter in place. Many sought refuge in bars and hotels, while public transport throughout the old town was shut down and police scoured the city.
"It is the hardest day for Austria in many years. We are dealing with a terror attack the severity of which, thank God, we have not experienced in Austria in many years," Interior Minister Karl Nehammer told a news conference.
Austria's capital had so far been spared the kind of deadly militant attacks that have struck Paris, London, Berlin and Brussels, among others, in recent years. Chancellor Sebastian Kurz said the "repulsive" act was "definitely a terror attack", but he could not say what the motive was.
Oskar Deutsch, the head of Vienna's Jewish community, which has offices adjoining the synagogue on a narrow cobbled street dotted with bars, said on Twitter that it was not clear whether the temple or offices were targeted but that they were closed at the time.
Rabbi Schlomo Hofmeister told London's LBC radio he was living in the compound of the synagogue. "Upon hearing shots, we looked down (from) the windows and saw the gunmen shooting at the guests of the various bars and pubs," he said.
"The gunmen were running around and shooting at least 100 rounds or even more in front of our building," he said.
Border checks were being reinforced, the Interior Ministry said, and children would not be required to attend school on Tuesday. Although people were urged to stay indoors Vienna Mayor Michael Ludwig told broadcaster ORF the city would run normally on Tuesday, albeit with a tougher police presence.
"According to what we currently know, at least one perpetrator is still on the run," Nehammer said.
"We have brought several special forces units together that are now searching for the presumed terrorists. I am therefore not limiting it to an area of Vienna, because these are mobile perpetrators," Nehammer earlier told ORF.
Kurz said the army would protect sites in the capital so the police could focus on anti-terror operations. Speaking to ORF, he said the attackers "were very well equipped with automatic weapons" and had "prepared professionally".
Videos circulated on social media of a gunman running down a cobblestone street shooting and shouting. One showed a man gunning down a person outside what appeared to be a bar on the street housing the synagogue. Reuters could not immediately verify the videos.
Condolences poured in from around the world, with top officials from the European Union, France, Norway, Greece and the United States expressing their shock at the attacks.
President Emmanuel Macron of France, which has seen two deadly knife attacks in Paris and Nice in recent weeks, issued a statement expressing shock and sorrow.
"This is our Europe," he said. "Our enemies must know with whom they are dealing. We will not retreat."
French officials have ramped up security since the attacks in Paris and Nice, which had suspected Islamist motives. Macron has deployed thousands of soldiers to protect sites such as places of worship and schools, and ministers have warned that other Islamist militant attacks could take place.
U.S. President Donald Trump said in a tweet 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 U.S. stands with Austria, France, and all of Europe in the fight against terrorists, including radical Islamic terrorists."
Austrian officials have not released any details about the attackers or the potential motive for the Vienna attack, although Kurz said an anti-Semitic motive could not be ruled out.
Democratic presidential candidate Joe Biden condemned what he called a "horrific terrorist attack," adding, "We must all stand united against hate and violence."
In 1981, two people were killed and 18 injured during an attack by two Palestinians at the same Vienna synagogue. In 1985, a Palestinian extremist group killed three civilians in an attack at the airport.
Nehammer is due to hold a news conference on the situation at 6 a.m. (0500 GMT) on Tuesday.