The German police are conducting house searches in several places because of the possible connection of four people with the attack in Vienna.
According to the police, they are looking in Onsnabrück, Kassel and near Hamburg.
“There is no suspicion that the suspects are directly involved in the attack, but we have evidence that the people we are looking for have links to the attacker,” a police spokesman said.
The perpetrator, a 20-year-old radicalized Muslim, shot four people on Monday evening and wounded more than 20 others in the heart of Vienna.
He was killed by the police. The attack was the work of one person and was claimed by the terrorist movement Islamic State (IS).
Fifteen people have been arrested in Austria on suspicion of involvement in the attack or of links to the terrorist. In Switzerland, two people are also detained on the same doubt.