Former United Nations judges and investigators have urged U.N. human rights chief Michelle Bachelet to look into the 1988 “massacre” of political prisoners in Iran, including the suspected participation of Iran’s current president, Embrahim Raisi, at the time.

The open letter, obtained by Reuters on Thursday, was signed by 460 persons, including Sang-Hyun Song, a former president of the International Criminal Court (ICC), and Stephen Rapp, a former US ambassador for global criminal justice.

Raisi, who assumed office in August, is subject to US sanctions because of his alleged role as one of four judges who supervised the 1988 executions, according to the US and campaigners. On Thursday, his office in Tehran had no reaction.

Iran has never admitted that mass executions occurred under the reign of Ayatollah Ruhollah Khomeini, the revolutionary leader who died in 1989.

According to Amnesty International, about 5,000 people have been executed, with the organization stating in a 2018 study that “the true figure might be higher.”

“The perpetrators continue to enjoy impunity. They include the current Iranian President Ebrahim Raisi and judiciary chief Gholam-Hossein Mohseni Ejei,” said the open letter. Ejei succeeded Raisi as head of Iran’s judiciary.

Raisa, when asked about activists’ allegations that he was involved in the killings, told a news conference in June 2021: “If a judge, a prosecutor has defended the security of the people, he should be praised.” He added: “I am proud to have defended human rights in every position I have held so far.”

The letter, organized by the British-based group Justice for Victims of the 1988 Massacre in Iran, was also sent to the U.N. Human Rights Council, whose 47 member states open a five-week session on Feb. 28.

Other signatories include previous U.N. investigators into torture and former foreign ministers of Australia, Belgium, Canada, Italy, Kosovo and Poland.

Javaid Rehman, the U.N. investigator on human rights in Iran who is due to report to the session, called in an interview with Reuters last June for an independent inquiry into the allegations of state-ordered executions in 1988 and the role played by Raisi as Tehran deputy prosecutor.