The 20-year-old was sentenced to 22 months in jail in April last year for attempting to travel to Syria to join the group, but was released early in December.
Swiss police said they arrested a 18-year-old and 24-year-old – both Swiss citizens – in the city of Winterthur on Tuesday afternoon over possible links to the main suspect.
Karin Keller-Sutter, Switzerland’s justice minister, said the men were “obviously friends” with Kujtim Fejzulai, the 20-year-old suspected of carrying out the attack in Vienna.
She added the men had met in person, without saying when.
“The extent to which there was a connection between the two arrested persons and the alleged assassin is currently the subject of ongoing clarifications and investigations which are being carried out by the responsible authorities,” Zurich cantonal police said following the arrests.
Fejzulai – who was shot dead during the attack – pledged allegiance to Isis on Instagram hours before the rampage took place.
A gunman opened fire on crowded bars in Vienna on Monday evening. Police said there were six shooting locations, including one near the city’s main synagogue.
An elderly man and woman, a young passer-by and a waitress were killed, and 22 people – including a policeman – were injured, according to Karl Nehammer, the Austrian interior minister.
He said footage of the incident filmed on numerous mobile phones showed no evidence of a second gunman, although the possibility had not been completely ruled out.