A man in Germany has been sentenced to life in prison after being found guilty of poisoning his co-workers' sandwiches over several years by mixing heavy metals into their lunches.

The 57-year-old man, named only as Klaus O, was arrested back in May after surveillance video showed him opening a colleague's lunchbox at a business in western Germany and putting a suspicious powder inside.

Bielefeld police said at the time the powder contained lead acetate "in amounts that would have been sufficient to cause serious organ damage," and a search of the suspect's home uncovered further additional dangerous substances.

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A judge gave the 57-year-old the jail sentence Thursday after convicting him of attempted murder. Authorities initially were probing if he was linked to a string of 21 deaths over two decades, but later said he was involved with at least 3 poisonings.

Prosecutors said the man laced colleagues' lunches with chemicals such as mercury and lead acetate between 2015 and 2018. Two victims, aged 27 and 67, suffered chronic kidney damage from poisoning with lead and cadmium with a heightened risk of cancer for the rest of their lives. A 23-year-old trainee fell into a coma after ingesting mercury and has permanent brain damage.

A man who worked with Klaus for 30 years told Bild last year his kidneys failed suddenly, and he is unable to drink more than a few ounces of liquid a day without suffering severe cramps.

“Doctors could not explain why I got so sick,” the man, identified as Udo B, told Bild at the time.

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During the trial, a psychologist testified the 57-year-old wanted to "experiment" with toxins and how they affected his co-workers, Deutsche Welle reported.

Klaus had "seemed to me like a scientist who was testing substances on a guinea pig," he told the court.

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

Life sentences in Germany usually can be reduced after 15 years, but the judge ordered O. ineligible for such a reduction, saying he was inclined to commit more crimes and a danger to the public.

The Associated Press contributed to this report.