Mengistu has been living in Zimbabwe since 1991, where President Robert Mugabe is unlikely to force him to leave, the Voice of America said.
The government of Ethiopia charged that Mengistu killed thousands of students and other dissidents, beginning with a purge after he overthrew Emperor Haile Selassie in 1974.
Michael Clough, a U.S. lawyer and observer of Ethiopia, told VOA that the current regime in the country has also committed human rights abuses.
"The biggest problem with prosecuting Mengistu for genocide is that his actions did not necessarily target a particular group," Clough said. "They were directed against anybody who was opposing his government, and they were generally much more political than based on any ethnic targeting. In contrast, the irony is the Ethiopian government itself has been accused of genocide based on atrocities committed in Gambella."