A European ISIS terrorist cell met the Vienna attacker in Austria three months before he committed the atrocity which left four people dead and 22 wounded.
Austria's security chief said the convicted terrorist Kujtim Fejzulai held a summer meeting with people from Germany, who were under observation, and two men from Switzerland who have since been arrested.
Fejzulai, 20, wore a fake suicide vest and opened fire on innocent bystanders in a nine-minute rampage in Vienna last week before police shot him dead.
"A meeting took place in Vienna among the people (you) addressed from Germany and Switzerland but there were also people present at the meeting with the later assailant who were arrested in the context of the investigation," Director General for Public Security Franz Ruf told a news conference on Monday.
"It was a larger circle of people that met. Some spent the night, the rest then left."
Austria has acknowledged that "intolerable mistakes were made" by its security services in the handling of intelligence on the attacker.
It was revealed convicted ISIS sympathiser Fejzulai had attempted to buy weapons in Slovakia in July and had met people from Germany who were under observation.
It is thought two men arrested in Switzerland in connection with the atrocity had travelled to Vienna between July 16 and July 20 to meet the attacker.
Last week, Austria launched an inquiry into failings by its security services and the head of Austria's main domestic intelligence agency for the city of Vienna stepped down pending an investigation.
Austrian intelligence is "traditionally weak and must be strengthened" as part of an ongoing overhaul, Mr Ruf said.
Austrian 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 said they had identified one of them as “probably” being 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.
Last week police carried out 18 raids across Austria and arrested 15 people, aged between 18 and 28.
Mr Ruf said seven of the individuals held have criminal convictions, four for terrorism-related offences.
Fejzulai, a dual citizen of Austria and North Macedonia, was 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.