GENEVA: Prominent former U.N. judges and investigators have called on U.N. human rights boss Michelle Bachelet to investigate the 1988 "massacre" of political prisoners in Iran, including the alleged role of its current president, Ebrahim Raisi, at that time.

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

Raisi, who took office in August, is under U.S. sanctions over a past that includes what the United States and activists say was his involvement as one of four judges who oversaw the 1988 killings. His office in Tehran had no comment on Thursday.

Iran has never acknowledged that mass executions took place under Ayatollah Ruhollah Khomeini, the revolutionary leader who died in 1989.

Amnesty International has put the number executed at some 5,000, saying in a 2018 report that "the real number could 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.

Raisi, 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, organised 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.