Terror attack in Vienna
Terror attack in Vienna Reuters

Eight suspects detained following Monday night's terrorist attack in Vienna had previous convictions, including for terror offenses, the Austrian interior minister said Thursday, according to AFP.

Meanwhile, the investigation extended to Switzerland and a second, unnamed country, the report said.

In the wake of the attack, in which an Austrian-born man shot and killed four people in a popular nightlife area of central Vienna, police arrested a total of 16 men.

four of them had been convicted for terrorism-related offenses, two for violent crime offenses, and two for an attempted "honor killing", Interior Minister Karl Nehammer told a press conference Thursday.

"We have had intensive cooperation with the FBI," which provided Austrian authorities with "valuable information", Nehammer added, without giving further details.

The investigation has also led to Switzerland, where prosecutors have confirmed that two Swiss men aged 18 and 24 who were arrested Wednesday had already been the targets of criminal cases over terrorism offences.

Authorities in another country are also investigating "direct links to the perpetrator," according to Nehammer, but added he wasn't able to name the country at this stage because of ongoing investigations.

The suspect in the attack, identified as 20-year-old Kujtim Fejzulai, had a previous conviction for trying to join the Islamic State (ISIS) group in Syria and had been given early release in December.

On Wednesday, it was reported that Slovakian intelligence told their Austrian counterparts in July that the Vienna terrorist had tried to buy ammunition there, but mistakes were made by Austria in dealing with that information.

Security officials on Thursday addressed the criticism that they did not follow up on a warning from Slovakia regarding Fejzulai.

Vienna police chief Gerhard Puerstl said the suspect was not initially clearly identifiable in photos sent from Slovakia.