Vienna's counter-terrorism chief has been suspended after the country admitted "intolerable mistakes" in the lead-up to the gun rampage that left four dead.
Interior Minister Karl Nehammer said police should have considered Kujtim Fejzulai, 20, to be a greater threat and monitored him more closely.
The convicted ISIS supporter opened fire in central Vienna on the country’s last night before lockdown on Monday.
Fejzulai was released from jail in December last year after securing early release by “fooling” his way through a deradicalisation programme.
Mr Nehammer said Vienna’s counter-terrorism chief had asked to be suspended while an investigation takes place.
Officials also shut down two mosques attended by the shooter after suspicion they fuelled his radicalisation.
“From our point of view intolerable mistakes were made," Mr Nehammer said.
Austria already admitted it fumbled Slovakian intelligence that suggested the gunman had attempted to buy ammunition there.
People under German surveillance had also met Fejzulai in Austria, police said.
Vienna police chief Gerhard Puerstl admitted this information alone could have been crucial to stopping the gunman.
All threat assessments were now being reviewed.
Two Vienna mosques attended by the gunman were ordered to close on Friday after intelligence agencies said the places of worship “furthered the attacker's radicalisation” after his release from jail.
Only one of the mosques was officially registered as such, officials said.
The Islamic Religious Community of Austria said the registered mosque was being shut because it had broken rules over "religious doctrine and its constitution", as well as national legislation governing Islamic institutions.
Fifteen people were arrested after sweeping raids on the day after the attack but officials said only eight remain in custody.
They are "strongly suspected of having contributed to or committed the crime of involvement in a terrorist group or criminal organisation," prosecutors said.
Police in Germany also searched homes and businesses linked to four people believed to have had ties to the shooter.
Fejzulai killed a waitress, an elderly man and woman, and a male passer-by.
While terrorist attacks in Vienna are rare, Monday’s shooting occurred near the Stadttempel synagogue – the same location where a Palestinian gunman killed two people in 1981.