If confirmed, Friday's killings in the northern city of Linz -- in which an 85-year-old woman's throat was slit and her 87-year-old husband was stabbed and beaten to death -- would be the first Islamist attack in Austria, which had so far avoided the jihadist assaults seen elsewhere in Europe.
The killings "clearly had an Islamist background", Sobotka told a press conference in Vienna yesterday, adding that the 54-year-old suspect, who handed himself in to police, "is clearly a radicalised Muslim".
Police had said Tuesday that the murders did not initially appear to be an Islamist attack, with investigators focusing on the theory that the suspect harboured resentment against society as well as Austria's far-right Freedom Party (FPOe).
He knew the victims, having regularly delivered their shopping from a shop run by his wife, and allegedly believed the elderly couple had links to the far-right.
More From This Section
But Sobotka told a press conference that the investigation had taken a turn after searches of the suspect's home and examination of his electronic data.
He declined to give further details on the evidence against the Tunisian, who has lived in Austria since 1989, or what he had said to investigators.
Chancellor Christian Kern urged an investigation "without delay" into the extremist threat to determine "where similar risks exist and how to combat them efficiently".
While Austria has until now been spared the major attacks that have hit other European countries including France, Britain and Germany, a relatively high number of Austrians have left to wage jihad abroad.
The interior ministry estimates that 300 have quit Austria or tried to leave in order to fight in Syria or Iraq -- a figure that is proportionately high in a population of 8.7 million.