MEXICO CITY, Mexico -- A truth commission investigating one of Mexico's worst human rights tragedies said Thursday that military personnel bore responsibility, either directly or through negligence, in the disappearance of 43 students in 2014.

"Their actions, omissions or participation allowed the disappearance and execution of the students, as well as the murder of six other people," said the commission's head, deputy interior minister Alejandro Encinas.

The teaching students had commandeered buses in the southern state of Guerrero to travel to a demonstration before they went missing.

According to an official report presented in 2015 by the government of then-president Enrique Pena Nieto, they were arrested by corrupt police and handed over to a drug cartel.

The cartel mistook the students for members of a rival gang and killed them before incinerating and dumping their remains, according to that report, which did not attribute responsibility to the military.

Those conclusions were rejected by relatives as well as independent experts and the office of the United Nations High Commissioner for Human Rights.

Encinas said that further investigations were necessary to establish the extent of participation by army and navy personnel.

"An action of an institutional nature was not proven, but there was clear responsibility of members" of the armed forces stationed in the area at the time, he said.

President Andres Manuel Lopez Obrador said in March that navy members were under investigation for allegedly tampering with evidence, notably at a garbage dump where human remains were found, including those of the only three students identified so far.

He denied an accusation by independent experts that Mexican authorities were withholding important information about the case, which shocked the country and drew international condemnation.