A German man has been sentenced to life in prison for poisoning his co-workers' sandwiches with mercury and other substances over several years, leaving one in a coma and two others with serious kidney damage.

A judge at the regional court in Bielefeld, about 205 miles west of Berlin, found the 57-year-old defendant guilty Thursday of attempted murder and gave him the maximum possible sentence.

The man, identified only as Klaus O. in line with German privacy rules, was arrested in May after surveillance video showed him putting a suspicious powder on a colleague's sandwich at a business in the town of Schloss Holte-Stukenbrock. Police said the powder contained lead acetate "in amounts that would have been sufficient to cause serious organ damage."

A 57-year old man found guilty of poisoning his co-workers' sandwiches with mercury, lead acetate and other chemicals over several years hides his face during a court appearance in Bielefeld, Germany, Thursday. Friso Gentsch / AP

Authorities initially were probing if he was linked to the premature deaths of 21 people going back to the year 2000, but It has still not been proven that any of the dead individuals were poisoned, the German news agency DPA reported.

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When authorities searched his home, they found a primitive chemistry laboratory in the basement and a substance that Judge Georg Zimmermann described as "more dangerous than all combat agents used in World War II."

O. refused to speak during his trial, and his motives remain unclear. Prosecutors believe he wanted to see his colleagues' physical decline.

Two of them, a 27-year-old and the other age 67, suffered chronic kidney damage from poisoning with lead and cadmium. Both men face a heightened risk of cancer.

A 23-year-old trainee fell into a coma after ingesting mercury and has permanent brain damage.

German news agency Deutsche Presse-Agentur (DPA) quoted the judge saying the court considered the crimes to be as serious as homicide. Zimmermann ordered that O. should remain in prison after completing the life sentence — which in Germany typically means serving 15 years — because he remains a danger to the public.

The defendant's lawyers plan to appeal.