The employees, two young women and a young man mixed the antifreeze in their 39-year-old colleague's morning coffee, causing chemical burns in his nose and throat.
“It is a ghoulish joke gone wrong,” police officer Fritz Bezikofer told The Local on Friday. “It was just what we call Jux und Tollerei", or good-natured fun, he added.
The younger staff members began a series of pranks in early November, thinning their coworker's coffee with all kinds of “ingredients” such as cola, orange juice and "nose and throat slime." During the course of the month stepped up the intensity of the pranks by urinating in the man's thermos bottle, which he kept in a meeting area.
But their final trick has police in the German state of the Baden-Württemberg investigating them for poisoning and causing bodily harm to the man, who is now on sick leave after drinking the antifreeze.
Police however insist that the incident was not social mobbing. “They all got along great,” Bezikofer said. “He was away from work very often, and they used that to play pranks. They just went too far and the situation escalated,” he adds.
Police are also checking into whether coworkers contaminated food at the restaurant.