One manager called Klaus “​​conspicuously inconspicuous” (via Deutsche Welle). Another of Klaus’s victims, a man he had worked with for three decades, said (via The Washington Post), “He always kept to himself, did not speak and had no friends. I had no problem with Klaus, and I accepted that he does not want any contact. There was never an argument.”

But Klaus’ co-workers were having lots of weird illnesses and symptoms. One startling case involved Nick N., a 23-year-old employee who was a summer apprentice with the metal company. He began to have lasting bouts of fatigue and noticed that his hands were starting to go numb with no explanation. Then one day, he started vomiting and was struggling to walk. He went to a hospital, where he fell into a coma and did not reawaken. After three years, Nick N. died at age 26. And he was far from Klaus’ only victim. 

Klaus might have gotten away with these heinous and extreme poisonings if it weren’t for security footage (per The Cut).