First his fingers turned numb. Then it was the lingering fatigue. And then, one day in the summer of 2016, the 23-year-old employee at a German metal-fitting company came home from his summer apprenticeship and vomited. He could barely walk.

His last steps would be through the hospital doors, as German newspaper Der Spiegel reported. The young man, identified only as Nick N., slipped into a coma, and soon, the cause of his sudden decline would only raise more questions: It was mercury poisoning.

For months, no one could figure out how Nick might have come in contact with the deadly toxin — until two more co-workers at the factory fell mysteriously ill.

One of them, Simon Radtke, suspected the sandwiches, according to Der Spiegel.

He started noticing sprinkles of white and brown powder on the bread during his lunch break in the spring of 2018. The factory, ARI Armaturen, in the industrial town of Schloss Holte-Stukenbrock, decided to install a surveillance camera. And then everything became clear.

Everything — Nick’s coma, the dirty sandwiches — could be traced to one man: a toolmaker identified only as Klaus O., who never spoke to anyone and who had been sneaking into the break room, pulling vials of lead and mercury compounds from his pocket and poisoning his co-workers for no discernible reason.

Now, more than three years after Nick N. collapsed into a coma and never woke up, Klaus O.'s poison killed him, the Bielefeld public prosecutor’s office confirmed to The Washington Post on Friday. Nick N. was 26.

Klaus O., who worked at the factory for 38 years, was convicted of attempted murder and bodily injury in March in three cases, including that of Nick and Radtke, after being captured on video adding mercury and lead compounds to the sandwiches. He was sentenced to life in prison. But the toolmaker’s case is far from closed. Nick N. is now at least the 22nd employee at ARI Armaturen to have died before reaching the age of retirement in the last 20 years — raising fears that Klaus O., 57, may have been a serial poisoner all along, prosecutor Moritz Kutkuhn told The Post.

“It was suspicious to us that there were 21 deaths at that same company,” Kutkuhn said, “and alarming that they all had medical problems.”

Klaus O., whose full name is withheld under German privacy code, is appealing his conviction. An attorney who represented Klaus during his trial did not immediately respond to a request for comment but argued at trial that prosecutors could not prove that Klaus O. poisoned Nick N., Der Spiegel reported.

Last year, after tallying all of the factory workers’ untimely deaths in the last two decades, police in Bielefeld began exhuming their bodies to investigate whether poison was present in their remains. The workers died of cancer or heart attacks, which authorities said possibly could have been caused by heavy metal poisoning that went undetected.

Kutkuhn said on Thursday that poisoning has been ruled out in nine cases, while 12 are still under investigation.

Klaus began working at the ARI Armaturen company as a locksmith apprentice in 1979, Kutkuhn said.

His co-workers remembered him as a loner who did not care for small talk and did not care to say hello, Der Spiegel reported. His manager called him “conspicuously inconspicuous."

“He always kept to himself, did not speak and had no friends,” another co-worker told Bild, a German newspaper. “I had no problem with Klaus, and I accepted that he does not want any contact. There was never an argument.”

Klaus had no criminal record and showed an exemplary work ethic at the factory, his manager said. He rode his bike 12 miles to work every day and returned to a comfortable home where he lived with his wife and two kids, Der Spiegel reported.

But in the basement, Klaus was experimenting with “substances that were more dangerous than warfare agents that were used in World War II,” the presiding judge, George Zimmerman, said during his trial, Der Spiegel reported. When police raided his home, they found instructive reading material: “Reports of the German Chemical Society,” from 1913, and “Experimental Tuberculosis Research,” from the University of Kyoto in 1941. Klaus was ordering the chemicals from the Internet, police would soon learn, and then bringing them to work.

Radtke said he and his co-workers started experiencing strange symptoms in at least 2015 after eating their sandwiches, Der Spiegel reported. For two years, Udo B. and Radtke felt themselves growing increasingly fatigued. Finally, in 2018, Radtke spotted the unfamiliar powder dirtying his bread and persuaded the company to set up a video camera. Next he showed the bread and the tape to the police.

Klaus was arrested on May 16, 2018, with a glass vial of lead acetate in his pocket, Der Spiegel reported.

During his trial, a psychologist determined that Klaus appeared to be completely mentally sound, and that there was “nothing in Mr. O that excuses him from a psychiatric perspective.”

“The problem is,” Zimmerman said, according to Der Spiegel, “we don’t know what Mr. O thinks about his actions.”

Klaus, or “Mr. O,” would never say. The psychologist said he did not appear to gain any sadistic pleasure from the poisonings, “but he did not show pity either,” Der Spiegel reported. He said Klaus “seemed to me like a scientist who was testing substances on a guinea pig,” the psychologist said, according to the German broadcaster DW.

On the witness stand, Radtke and Udo B. had only one question: why? They said it had tormented them, never finding any reason for their suffering or why Klaus had chosen them. Radtke once could run nearly a dozen miles, Der Spiegel reported, but now could barely walk up the stairs. Last March, the men said they suffered from kidney failure. They could do little else but go to dialysis and rest on the couch.

Nick N.'s parents took the witness stand, too. His father said he didn’t want to live anymore but knew he couldn’t die. His mother said she wished Klaus could feel all of her suffering — but not his children.

“They can’t help that their father is a madman,” she said, Der Spiegel reported.

Klaus raised his eyebrows, one of the only reactions he would offer nearly the entire trial, the paper reported.

Except to identify himself to the judge, throughout the trial, Klaus barely spoke at all.