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.
Botzenhardt said police were investigating "all possible avenues."
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 that the suspect appears to have had mental health problems.
"We now know it was in all likelihood a lone perpetrator, a German," Reul also told reporters, adding that there was no evidence of any link to Islamist militancy.
Meanwhile, Muenster police chief Hajo Kuhlisch said there were "no indications of a political motive."
Rather, "the motive and origins [of the attack] lie within the perpetrator himself," he added.
The incident occurred at a time of a heightened concern in Europe, following a series of Islamist attacks using vehicles to kill pedestrians in Britain, France, and Sweden.