German investigators were trying to work out why a 48-year-old man drove a van into a group of people in the western city of Muenster on April 7, killing two people before shooting himself dead.
"So far there are no indications of a possible background for the crime," prosecutor Martin Botzenhardt said in an April 8 statement.
The van plowed into people seated outside a restaurant in Muenster's old town, killing two and injuring 20 before the driver of the vehicle shot and killed himself inside it.
Six of the injured were in severe condition.
Herbert Reul, the interior minister of North Rhine-Westphalia state where Muenster is located, said the driver was believed to be a German citizen.
Local media identified him as an industrial designer living in Muenster who had been suffering from psychological problems, but police did not confirm this.
Authorities said there was no indication it was an Islamist attack.