Victoria state police do not believe the incident was terrorism-related.
Anis Amri said that ISIS supporters would seek revenge on what he called "crusaders" in Europe.
The brothers are thought to have planned an attack on a mall close to the Dutch border.
If we allow other people to strip away our values, we will be doing the terrorists' jobs for them.
German authorities have issued a Europe-wide arrest warrant for Amri, 24.
Several plots have been planned by refugees recently settled in Germany.
Police released the main suspect on Tuesday, citing a lack of evidence.
The truck-ramming in the German capital left at least 12 people dead and 48 injured.