By Stefan J. Bos
German police said the men targeted Friday were not considered suspects, but they contacted the attacker, and two met him in person.
Several suspects are still held in Austria, and officials are following up leads in Switzerland too.
The gunman went on a rampage in Vienna on Monday night, opening fire in six places in the center. Two women and two men were killed, and 23 were wounded in Monday's terror attack, for which the Islamic State group claimed responsibility.
Police shot the gunman dead. They found that the attacker named as Kujtim Fejzulai had both Austrian and Macedonian citizenship.
The shootings started around Vienna's main synagogue, recalled Rabbi Schlomo Hofmeister. "I heard the shootings on the street. In the beginning, I thought it was fireworks, which can happen at this time of year," he said. "But then I saw an attacker with a gun was shooting at the guests in the nearby bars and restaurants in the area."
Austria's Interior Minister Karl Nehammer said authorities are trying to track down the 20-year-old killer's contacts. He claimed that they were dealing with "a violent criminal who was clearly intensely active in the Islamist political network, a sympathizer who took on their ideology."
But the minister has admitted that a warning from Slovakia last summer about the attacker was not followed up. In Slovakia, police revealed they had tipped off Austrian authorities about "suspects from Austria" trying to buy ammunition in July.
He reportedly failed to buy bullets as the gunman had no license.
It has also emerged he was released early from a jail sentence last December for trying to join jihadists in Syria.
The shootings have underscored broader fears of Islamic terror attacks in Europe following the recent stabbing of three people in a church in the French city of Nice and the beheading of a teacher in a Paris suburb.