The sites in Osnabrück, Kassel as well as in the Pinneberg area that were searched belong to four people, who “are not believed to be involved in the attack,” said the federal criminal agency (BKA).

“But there may be links to the alleged assassin,” it added on Twitter.

The searches were carried out on a request from the Austrian authorities and were aimed at “securing material that could be proof”.

“Two of the individuals are believed to have met the suspected assailant in July 2020 in Vienna,” the agency added.

The gunman, identified as 20-year-old dual Austrian-Macedonian national Kujtim Fejzulai, was killed by police after going on a shooting spree in Vienna on Monday evening that left four people dead.

Austrian police detained 14 people in the wake of the shooting, the first major attack in the country for decades and the first blamed on a jihadist.

Germany's Der Spiegel magazine had reported earlier this week that the Vienna attacker had made contact with German Islamists during an attempt to travel to Syria to join the Islamic State group.

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.

READ ALSO: Half of those arrested after Vienna attack 'had previous terrorism or violence convictions'