The police in the German city of Darmstadt announced on Tuesday that they have now identified the substance added to various drinks on the city's university campus over the weekend which led to the poisoning of six people.
The police did not immediately disclose which substance was involved in what they are assuming to be a malicious act.
On Monday, several people at Darmstadt's Technical University were taken to hospital with symptoms of poisoning including lethargy and discolouration.
During the police search of the university's Lichtwiese campus on Tuesday, no other contaminated items were found and no further symptoms of poisoning were reported.
The police suspect several milk packages and water containers were contaminated with the noxious substance over the weekend. The "acrid smell of the adulterated liquids" was particularly noticeable, they said.
Police have urged people on campus only to eat food that was stored safely, adding that the danger of poisoning was no longer acute.
Of the six people receiving treatment in hospital, one 30-year-old student remains in a critical condition.
