Austria failed to follow up on information from Slovak authorities that the gunman who carried out Monday's deadly attack had tried to buy ammunition, the country's interior minister has said.
Slovak police said suspects from Austria tried to buy ammunition in the country in summer and that they immediately warned Austrian police about the failed transaction.
They said in a Facebook post titled "Vienna attack not with Slovakia munition" that they would not comment on the matter further "so that we don't endanger the investigation in Austria."
Austrian Interior Minister Karl Nehammer said on Wednesday that the information had been investigated by the domestic intelligence agency (BVT) but no action was taken beyond this.
"Something obviously went wrong in communication," he told the media, saying he wanted to organise an inquiry to look into the matter.
A man killed four people in a shooting rampage in Vienna, armed with an automatic weapon, a pistol and a machete.
He had sought to join the so-called Islamic State (IS) and most likely acted alone, Austrian authorities said on Tuesday.
Gunman shot dead
Authorities have named the gunman, who was carrying an assault rifle and a fake suicide vest, as Austrian-North Macedonian dual national Kujtim Fejzula.
He was shot dead by police.
The 20-year old had previously served time in prison following an April 2019 conviction for an attempt to travel to Syria to join IS.
He was granted early release from his 22-month jail term in December under juvenile law, authorities said.
Authorities had initially feared that multiple gunmen were involved in the attack, but Interior Minister Nehammer told reporters early in the afternoon that evidence showed no indication there had been other assailants.
He added that 14 people close to the suspected perpetrator had been detained and that 18 properties in and around the Austrian capital had been searched.
Two men, aged 18 and 24, were also arrested near Zurich, Swiss police have said.
Four people — two men and two women — were killed in what Chancellor Sebastian Kurz a "repulsive terrorist attack" as residents of the capital enjoyed a final night out before the city entered a partial coronavirus lockdown. One of the victims was a German national, Germany's Foreign Minister, Heiko Maas, confirmed on Twitter.
A further 22 people were injured, some of the "seriously", police said. Austrian news agency APA reports that Vienna’s hospital service said seven people were in life-threatening condition.
A minute of silence was held across the country on Tuesday afternoon to commemorate all the victims.
Kurz has convened the country's national security council to discuss what should happen next and stressed that the fact that the attacks were carried out by Islamists did not mean that Austrians "should condemn all members of a religion".
"It is a fight between the people who believe in peace and the terrorists," he said at a news conference.
He said it was clearly an "Islamist" attack and said that the attackers were motivated by hatred of Austrian democracy and its way of life.
What do we know about the attack?
It began at just after 8 pm CET on Monday night in a street close to Vienna's main synagogue and, speaking Monday, Kurz said the attackers may have had anti-semitic motives.
A witness to the attack, Eveline, told AP that she and others had taken refuge in a local hotel.
"Suddenly the shooting started, at first we did not know what it was ... Then there was shooting again, but closer, so we started to run away," she said.
Oskar Deutsch, a Jewish community leader, said it was too early to say whether the temple was one of the targets but confirmed there had been "a shooting in the immediate vicinity of the city temple."
Deutsch said that both the synagogue in Seitenstettengasse and an office building at the same address were closed at the time of the attack.
Nehammer said that the army was guarding key locations in the city as hundreds of police hunted for the remaining gunmen.
The minister told people in Vienna to stay indoors and avoid the city centre and encouraged parents not to send their children to school on Tuesday.
The attack has echoes of those in London in 2017 and Paris in 2015 when attackers roamed the streets of the two cities armed with knives and guns, killing maiming civilians in restaurants and bars.
What have we heard from the scene?
Vienna's Chief Rabbi Schlomo Hofmeister told broadcaster LBC in a radio interview that he was in the Seitenstettengasse synagogue's compound and saw multiple gunmen firing into bars and restaurants.
"The gunmen were running around, shooting at least 100 rounds or even more, in front of our building," he added.
He said he "doubted it was an attack on the synagogue, adding: "At this time of night there is no activity taking place in the great synagogue, we don't really know, however, what's going on."
Police asked social media users not to share photos or videos that purported to show the incident because it "endangers both emergency services and the civilian population".
In videos shared on social media, people were pictured running as what appeared to be gunshots could be heard.
Czech police said Monday evening that they were "carrying out random checks on vehicles and passengers" at the country's border with Austria after the attack.
"Police officers exercise increased supervision over the most important Jewish buildings in the Czech Republic," the force added in a tweet. "We assure the public that the measures taken are exclusively preventive in nature."
And Germany has put controls in place at the border too, which a German police spokesman said were a "tactical priority".
"Europe strongly condemns this cowardly act that violates life and our human values," President of the European Council Charles Michel wrote on Twitter.
"I am deeply shocked by the terrible attacks in Vienna tonight. The UK’s thoughts are with the people of Austria - we stand united with you against terror," said UK Prime Minister Boris Johnson.
"Terrifying, disturbing reports reach us this evening. Even if the extent of the terror is not yet known, our thoughts are with the injured and victims in these difficult hours. We must not give way to the hatred that is intended to divide our societies," the German foreign ministry said in a tweet.
"We the French share the shock and grief of the Austrian people," French President Emmanuel Macron wrote. "After France, an ally country has been attacked. This is our Europe. Our enemies must know who they are dealing with. We will not give up."
France has been struck by two violent attacks in a matter of weeks; a knifeman killed three people last week at a church in Nice and on October 16 a French teacher, Samuel Paty, was beheaded after showing cartoons depicting Islam's prophet in a class on freedom of speech.