AYOTZINAPA, Mexico — Four years after the disappearance of 43 students in southern Mexico, the case remains unsolved, international human rights groups said Monday, as they called on the next president to conduct a proper investigation.

The case of the missing students prompted global outrage and shook Mexico to its core, plunging President Enrique Peña Nieto’s approval ratings to new lows. At a ceremony on Monday at their school in Ayotzinapa, Guerrero, representatives from several rights groups said their disappearance had still not been explained.

The official account, disputed by international experts, is that the students were kidnapped by local police officers who turned them over to a drug gang. The gang killed them and burned their bodies in a nearby garbage dump, leaving no remains.

“The commission does not accept that narrative,” said Esmeralda Arosemena de Troitiño, rapporteur for Mexico at the Inter-American Commission on Human Rights. “No more talk about what they describe as ‘historic truth,’ because it hurts us, it outrages us and the families of the victims simply do not tolerate it.”