Austria says the two mosques, one unofficial, were attended by a lone wolf gunman who shot and killed four people on the streets of Vienna. It also admitted 'intolerable mistakes' in checks on attacker.
The Austrian government has ordered the closure of two mosques in the capital Vienna.
The move came on Friday after the government said these two places of worship were attended by a lone-wolf gunman who shot and killed four people in Vienna's city centre earlier in the week.
The shooting on Monday was Austria's first major attack in decades and its first blamed on a terrorist, identified as 20-year-old Austrian man 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 visited two Vienna mosques."
The two mosques are in Vienna's western suburbs.
One is the Melit Ibrahim mosque in the Ottakring district and the other is 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, without clarifying the mosques' role in Fejzulai's actions.
Mosques are public places where everyone who wishes to enter or attend can. The interior ministry did not specify who the attacker was in contact with at the mosque.
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.
The Islamic religious community IGGO said freedom of religion was an important value, but which must also be protected from within its own ranks. It said it worked with the authorities to close the mosque.
After Friday prayers at a mosque in Vienna, Imam Salim Mujkanovic condemned the attack and said: "Today it was a call to the youths who might not have had such a tight link (to the extremists) via the internet or personally, but there is a danger that they might be involved in this in the future."
Activists over the years have said closing down mosques in countries where Muslims live as a minority only furthers alienation.
After the Brussels bombings in March 2016, there was a move to merge or bring under the mosque into the state's fold instead of foreign sponsorship. The logic at the time was to assimilate the Muslim community who would also connect with Belgian culture in agreement with society and laws.
Such a form of assimilation, academics say creates a stronger bond between immigrant communities instead of acts such as the closure of mosques.
Slovakia information missed
Austria on Friday admitted "intolerable mistakes" in the handling of intelligence, 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 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 were in place.
Also on Friday, the Vienna prosecutor's department said 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 was a dual Austrian-Macedonian national who had previously been convicted for trying to join Daesh in Syria.