Regional rights activists investigating the case of 43 missing students in Mexico say they have been unable to interview military personnel.
They said soldiers who may have witnessed the students’ disappearance last September could only be questioned via a written questionnaire.
The team from the Inter-American Commission on Human Rights chose not to do this.
The Mexican government has not replied to the allegations.
The IACHR team also said that security videos containing visual evidence may have been destroyed.
Mexico’s interior ministry declined to comment on the accusation. It said in a statement that Interior Minister Miguel Angel Osorio and IACHR members had met the victims’ families on Friday.
The students – trainee teachers – were arrested by police in the southern city of Iguala and the government says they were then handed over to a drug lord who had them killed and their bodies incinerated.
The remains of only one of the students have been identified.
Investigators from the IACHR want to question soldiers from the battalion in Iguala that responded to events at the time the students clashed with police.
Relatives believe the soldiers may have failed to act as the police opened fire on the students and later bundled them onto a bus.
The IACHR has been investigating the case at the request of the victims’ families, with the agreement of the government.
A final report from the commission is due in September.