Four convicted terrorists are among those being held by officers after four people were killed and 22 others wounded in a terrorist attack on Monday.
Wearing a fake suicide vest, Kujtim Fejzulai, 20, an ISIS sympathiser, opened fire on innocent bystanders in a nine minute rampage before police shot him dead.
Police have carried out 18 raids across Austria and arrested 15 people, aged between 18 to 28.
All those arrested have “immigration roots” and some do not have Austrian citizenship, officials said.
Fejzulai, a dual citizen of Austria and North Macedonia, had been jailed in April 2019 for trying to travel to Syria to join ISIS.
He was released early from his 22-month sentence last December after “fooling” his way through a deradicalisation programme.
"We are dealing with a violent perpetrator who was evidently intensely involved in the network of political Islam, of sympathisers, who took on their ideology," Interior Minister Karl Nehammer said on Thursday.
Director General for Public Security Franz Ruf said seven of the individuals being held have criminal convictions, four for terrorism-related offences.
Of those four, two were involved in violent assaults and two were for attempted so-called honour killings.
"One can see that all are to be linked to the radical Islamist milieu,” Mr Ruf added.
Switzerland has also arrested two men in connection with the attack, and Austria was in close contact with another unspecified country in its investigation, Mr Nehammer said.
Separately, German Interior Minister Horst Seehofer told parliament there was a connection between the attack and people in Germany who are "monitored around the clock".
Mr Ruf has called for an inquiry into failings by the security services.
Officials had been tipped off by their counterparts in Slovakia in July that two people using a car with Austrian licence plates had attempted to purchase assault rifle ammunition at a shop in Bratislava.
Mr Ruf acknowledged on Thursday that they had identified one of them as “probably” Fejzulai by October 16 — more than two weeks before the attack — and said an independent investigation would look into whether mistakes were made.
“This commission will look into the process and evaluate it objectively,” he said.
Vienna’s chief of police, Gerhard Puerstl, said he was confident the security services had done their job properly.
“I can guarantee you that we have a clear conscience," he said.
Mr Nehammer also said Austria was working “intensively” with the FBI and had received “very significant information” from the US agency.
Chancellor Sebastian Kurz told a special session of parliament on Thursday that Austria currently does not have all the legal means necessary to monitor and sanction extremists.
He warned that the government could not allow a repeat of the situation that led to the attack.
