Grace, whose husband resigned in November under pressure from the military, received the doctorate in 2014 amid allegations she did not study for it.
She was awarded the degree after just months of study. Former president Robert Mugabe was University of Zimbabwe (UZ) chancellor at the time.
Doctorates typically required several years of full-time research and writing.
Nguni said that there was no evidence linking the former first lady to the violation of university regulations.
“There is no evidence to probe Grace. Grace is not a University of Zimbabwe official. She does not know the rules of how people are granted PhDs.
“We are investigating those people who knew the regulations and who fraudulently did whatever they did… we are probing those university authorities who violated university regulations,” Nguni was quoted as saying.
This came amid reports that the anti-graft agency was investigating one of the UZ lecturers, who supervised Grace’s PhD thesis.
Zacc indicated that Professor Claude Mararike faced arrest once investigations were concluded.
“He (Mararike) is under investigation and he faces arrest once investigations have been completed,” Zacc spokesperson, Phyllis Chikundura was quoted as saying recently.
The UZ head Levi Nyagura was arrested also last Friday over the controversial awarding of the PhD to Grace.
Investigators at ZACC said they had arrested Nyagura over the suspected fraudulent awarding of a doctorate to the former first lady.