The Austrian government ordered the closure on Friday of two mosques in the capital, Vienna, frequented by the gunman who shot dead four people in the city centre earlier in the week.

The shooting on Monday was Austria's first major attack in decades. The attacker was later identified as 20-year-old Kujtim Fejzulai, who was killed by police.

Integration Minister Susanne Raab told a press conference that the government's religious affairs office “was informed by the interior ministry that Monday's attacker, since his release from prison, had repeatedly visited two Vienna mosques”.

The two mosques are in Vienna's western suburbs, one called the Melit Ibrahim mosque in the Ottakring district and the other being the Tewhid mosque in the Meidling area.

The BVT domestic intelligence agency “told us that the visits to these mosques furthered the attacker's radicalisation,” Raab said.

Only one of the mosques was officially registered as such, Raab said.

A statement from the officially-recognised Islamic Religious Community of Austria said one officially registered mosque was being shut because it had broken rules over “religious doctrine and its constitution”, as well as national legislation governing Islamic institutions.

Also on Friday the Vienna prosecutor's department told AFP that six of the 16 people detained since the attack have been released, with the rest remaining in custody as the probe into the attacker's circle continues.

The suspected gunman, dual Austrian-Macedonian national Fejzulai, had previously been convicted for trying to join the militant Islamic State group in Syria.

'Intolerable mistakes'

The government on Friday also admitted to “intolerable mistakes” in the handling of intelligence on the attacker, saying it could have considered him a greater threat and monitored him more closely.

The head of the main domestic intelligence agency for the city of Vienna, the Vienna Provincial Office for the Protection of the Constitution and Counter-Terrorism (LVT) was stepping down temporarily while an investigation was carried out, Interior Minister Karl Nehammer told a news conference.

“Obvious and from our point of view intolerable mistakes were made,” Nehammer said.

Austria had already admitted fumbling intelligence from Slovakia that the 20-year-old gunman had attempted to buy ammunition there.

“People in Germany who were being monitored by German intelligence stayed in Austria in the summer and also met the attacker here,” Vienna police chief Gerhard Puerstl said.

“These facts together with the findings that emerged from the information from Slovakia could have led to a different outcome regarding the assessment of the threat posed by the perpetrator,” he added.

Nehammer said all such threat assessments would be reviewed to ensure the right monitoring measures are in place.