France's highest court has blocked plans to extradite three Rwandans to stand trial in Kigali on charges related to the 1994 genocide.
The court ruled that the men could not be tried for a crime which was not legally defined at the time it was allegedly committed.
Genocide became a punishable crime in Rwanda under laws passed in 1996 and 2004.
Some 800,000 people, mainly ethnic Tutsis, were killed in the genocide.
Militias from the rival Hutu ethnic group was accused of committing most of the atrocities.
Fled to France
The Court of Cassation overturned a ruling given by the appeals court last year, approving the extradition of Claude Muhayimana and Innocent Musabyimana.
It also upheld a ruling by another court in September, rejecting the extradition of Laurent Serubuga, who was Rwanda's deputy army chief-of-staff when the genocide took place.
Mr Muhayimana, who holds French and Rwandan citizenship, is accused of taking part in the massacre of Tutsis in the western town of Kibuye.
Mr Musabyimana is alleged to have been involved in the killings in the north-western province of Gisenyi.
Last July, Mr Serubuga was detained in France following an international arrest warrant issued by a Rwandan court.
The Court of Cassation upheld a ruling by a lower court, in the northern town of Douai, that Mr Serubuga could not be tried retroactively for crimes that were not part of the penal code when they took place.
The three men - who are Hutus - deny involvement in the genocide.
They fled to France after a new government, dominated by Tutsis, took power in Rwanda at the end of the genocide.
France has denied allegations that it did not do enough to stop the genocide.
It has also been accused by Rwanda's government of not doing enough to help provide justice for the victims.