German police are trying to find out why some euro banknotes of various denominations have been disintegrating - and who is responsible.
The brittle notes were not reported to be forgeries
The notes are believed to have come into contact with sulphuric acid - perhaps during a cleaning process, police spokesman Michael Maasz said.
"They do not pose a risk to public health," he told the BBC News website.
Hundreds of the brittle notes have appeared in at least 17 German towns and cities, German media report.
The popular German daily Bild says a blackmail attempt is one of the possibilities that police are investigating.
The paper says the banknotes are not forgeries.
The first disintegrating banknotes appeared in June and July in Berlin and Potsdam, Bild said.
Since then, they have been found further afield, including in Duesseldorf, Karlsruhe, Kiel and Wuerzburg.
German media say 50-euro notes are most commonly breaking apart.