That's according to the state-owned company and a government minister on Sunday (January 8).
Eskom's head of security said in a statement that it could not comment further on the alleged cyanide poisoning incident, which took place in December.
Public Enterprises Minister Pravin Gordhan said there would be a thorough investigation and anyone found to be responsible would be charged.
Without giving details, Gordhan said an intense battle was taking place between "those who want South Africa to work and thrive and those who want to corruptly enrich themselves."
De Ruyter officially took office in January 2020 - leading a company-wide clampdown on corruption and organized criminal behavior, including the sabotage of power plants.
But on December 14, under political pressure, he resigned.
He had failed to solve a crisis at Eskom that has led to record power cuts in South Africa.
De Ruyter could not be reached for comment. Eskom's board chairman Mpho Makwana was also unavailable.
South African police did not immediately respond to a request for comment.
