Authorities in Germany launched a probe Wednesday into a string of 21 deaths over nearly two decades after the arrest of a man suspected of trying to poison a co-worker's sandwich.

The 56-year-old man, named only as Klaus O, was detained back in May after surveillance video showed him opening a colleague's lunchbox and putting a suspicious powder inside, German tabloid Bild reported.

"In the beginning we thought it was a misconceived prank between co-workers, and not a murder attempt," Tilo Blechinger, the manager for the metal fittings manufacturer ARI Armaturen, told the DPA news agency.

Bielefeld police said 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.

The 21 deaths since 2000, which were all employees at the company the man worked for, included a "remarkably high number of heart attacks and cancers," according to police.

Three other workers at the company in Schloss Holte-Stukenbrock, located south of Bielefeld, are also believed to have fallen ill from heavy metal poisoning.

One employee has been in a coma for two years, according The Times of London.

A man who worked with the suspect for 30 years told Bild his kidneys failed suddenly three months ago, and he is unable to drink more 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.

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The 56-year-old man said Klaus O was a close colleague but he didn't know anything about him.

“He always stayed by himself, did not speak and had no friends," the man said.

A spokeswoman for the Poison Information Center at the University of Bonn told Bild it was a “very unusual case," and that lead poisoning is difficult to detect due to symptoms that might be caused by other conditions.

Authorities are considering exhuming the bodies to determine the presence of metal, according to Deutsche Welle.

The Associated Press contributed to this report.