Austria has acknowledged there had been security failings leading up to the deadly gun attacked in Vienna by a convicted Islamic State sympathiser.
Interior Minister Karl Nehammer said intelligence services had received a warning from neighbouring Slovakia that the assailant had tried to buy ammunition, but that "a failure of communication" had followed.
The gunman, identified as 20-year-old dual Austrian-Macedonian national Kujtim Fejzulai, was killed by police after going on a shooting spree in Vienna on Monday evening that left four people dead.
Police detained 14 people in the wake of the shooting, the first major attack in Austria for decades and the first blamed on a jihadist.
They were "aged 18 to 28, from minority communities and some aren't Austrian citizens", Mr Nehammer said.
Police say "it's possible they supported" the gunman but their exact role remains unclear.
The authorities now say Fejzulai acted alone after initial fears more assailants could be at large.
Fejzulai had been convicted and sentenced to 22 months in prison in April last year for trying to travel to Syria and join the Islamic State (IS) group.
But he was released on probation in December and had been referred to organisations specialising in de-radicalisation programmes.
IS, which has claimed numerous attacks in Europe, said on Tuesday a "soldier of the caliphate" was responsible for the shooting.
The gunman opened fire indiscriminately in the historic centre of the city just hours before Austria imposed a coronavirus lockdown, when people were out in bars and restaurants enjoying a final night of relative freedom.
Security has been tightened in the city.
Flowers and candles were laid out at the scene of the attack, where chalk circles drawn on the ground by investigators to mark out shell casings were still visible.
Mr Nehammer told reporters that the BVT domestic intelligence agency had been warned by Slovakia that Fejzulai was attempting to buy ammunition.
"In the next steps there was clearly a failure of communication," he said.
He accused his far-right predecessor Herbert Kickl of being responsible for failings in the way the BVT operates during his one-and-a-half years in office until May 2019.
Mr Nehammer said he wanted a commission set up to look at the functioning of the intelligence agencies.
Chancellor Sebastian Kurz has described the decision to release Fejzulai as "definitely wrong".
"If he had not been released then the terror attack would not have been possible," Mr Kurz told public broadcaster ORF on Tuesday.
Austria's top security chief Franz Ruf told local media that at his last session of a publicly-funded de-radicalisation programme in late October, Fejzulai had condemned the recent jihadist attacks in France.
Mr Nehammer has said the attacker successfully "fooled" the programmes, and that raids at his home revealed plentiful evidence of his radical views.
He referred to a Facebook post in which Fejzulai posed with the Kalashnikov and the machete he used in the attack, together with IS slogans.
"Nobody would have thought him capable of something like this," Nikolaus Rast, the lawyer who represented Fejzulai last year, told AFP on Wednesday.
He also raised questions about possible oversights by the de-radicalisation programmes Fejzulai had attended.
"Without wanting to put the blame on someone, if they are the experts, why didn't they notice anything?" Mr Rast said.
"They must have had the most - and the last - contact with him."
The organisation running the programmes, DERAD, hit back at the criticism, insisting that it had never described Fejzulai as "de-radicalised".
The investigation is spanning several countries, with Switzerland making two arrests and North Macedonia, where Fejzulai has family roots, cooperating with Austrian authorities.
Swiss prosecutors said the two young men arrested were already the target of criminal cases for terrorism offences.
Mr Kurz on Tuesday called for an EU response to "political Islam", saying the ideology was "dangerous" for European freedoms and values.
A planned visit to Vienna by French President Emmanuel Macron on Monday to discuss the battle against terrorism and "political Islam" was downgraded to a video conference, Mr Macron's office said late last night.
The recent re-publication of cartoons of the Prophet Mohammed in France has caused new tension worldwide, sparking protests in some Muslim-majority countries and calls from several terror groups for their followers to take revenge.
France itself has seen a deadly assault on churchgoers in the Mediterranean city of Nice and the beheading of a schoolteacher near Paris.