Amnesty International called on UN Human Rights Council Tuesday to make Iranian authorities stop concealing the mass graves of victims of the 1988 “prison massacres.”

The group also urged an international investigation into the extrajudicial execution and enforced disappearance of thousands of dissidents, which according to the group amount to ongoing crimes against humanity.

In recent months, Iranian authorities have erected new high concrete walls around the Khavaran cemetery outside the capital Tehran, where the remains of several hundred political dissidents executed in secret in 1988 are buried.

The construction has sparked serious concerns that the authorities would destroy or tamper with the mass grave site away from public view as the site is no longer visible from the outside and its entrance is guarded.

“The Iranian authorities cannot simply build a wall around a crime scene and think that all their crimes will be erased and forgotten. For 34 years, the authorities have systematically and deliberately concealed and destroyed key evidence that could be used to establish the truth about the scale of the extrajudicial executions carried out in 1988,” said Diana Eltahawy, a deputy director at Amnesty International. “This abomination must end once and for all, and the UN Human Rights Council must urgently establish an independent investigative mechanism to uncover the truth.”

Around 4,000 prisoners are believed to have been killed. Most victims were members or supporters of the exiled Mujahedin-e Khalq Organization (MEK) as well as Marxists and other members of other leftists groups.