The assistant and waiter are suspected in the case because they were the only two employees at the restaurant who did not experience any symptoms, which included fever, sweating, splitting headaches and a very fast heartbeat, thinkSpain reported Wedesday.
The victims said they had been getting these symptoms for as long as six or seven years.
During an investigation of the kitchen, a bottle of Colme (calcium cyanamid) was found. Colme is prescribed to alcoholics and provokes an ill reaction if they drink.
The victims said they are now suspicious the assistant and waiter played a role in the death of the restaurant owner, who died suddenly of a heart attack in May 2011. An autopsy, however, was not preformed at the time of death and he was cremated.
The suspect has been accused of not getting along with his colleagues and often being involved in conflict and arguments, thinkSpain said.