Chief Public Prosecutor Robert Hartmann (center) addresses journalists on Tuesday.
Berlin, Germany CNN  — 

German police have opened an attempted murder investigation after seven people who had consumed food and drink at a university kitchen showed symptoms of poisoning on Monday.

Blue discolorations appeared on their limbs following the incident at Darmstadt Technical University, German police and the public prosecutor in the federal state of Hessen said in a press release Tuesday.

A police spokesman told CNN on Wednesday that six people were hospitalized, one of whom was a 30-year-old student who was initially in a life-threatening condition.

Darmstadt Technical University said in a tweet late Tuesday that all six were feeling better and had now left hospital.

One person did not need medical assistance.

Police found several milk and water containers contaminated with a hazardous substance.

The substance is still unknown and is being examined, police said. They noted the acrid smell of the liquids was particularly striking.

Police said the perpetrator must have had access to the kitchen between Friday 20 and Monday 22 August.

The Police and Steering Committee of Darmstadt University has warned people not to consume any food stored at the Lichtwiese campus where the incident took place.

Buildings at the university campus were searched by police but no other suspicious objects have so far been found.

Police said there was no imminent threat but urged everybody to only consume food they had control of at all times.