Workers at the post office centre noticed the blue powder falling out of a letter on a conveyor belt around 7 pm. They apparently thought nothing of it and promptly vacuumed up the mess. But shortly thereafter 10 workers complained of skin and eye irritation and were transported to a nearby hospital.
Eighty other workers were treated in the postal centre's cafeteria and the building was eacuated, police said.
Then just before 10 pm a similar powder was found on a nearby street, which police closed off for investigation.
Initial tests showed that the substance was “not a chlorine compound but likely a fertiliser,” a police spokesperson said.
Police believe that someone emptied the substance into a Neusäß letter box – between the street where the powder was found and the post office – around 4 pm before it was emptied. Its contents were then taken to the post office.
Police are searching for witnesses who noticed someone inserting the substance into the Deutsche Post letter box on Tuesday afternoon.
The prank may have been a nasty April Fool's joke, police said.
Work resumed at the post office around 11:15 pm.