The scratchy, echo-filled tape recording carries the voice of a man who once was in line to become Iran's supreme leader, talking about one of the darkest moments of the country's post-revolution history still not recognized by its government. The recording has Grand Ayatollah Hossein Ali Montazeri condemning Iran's execution of thousands of prisoners at the end of the country's bloody war with Iraq in 1988. He warns those gathered they've committed "the biggest crime in the history of Iran," while criticizing them for misleading the country's then-ailing leader Ayatollah Khomeini. The criticisms by Montazeri, who lived for years under house arrest and died of natural causes after Iran's disputed 2009 presidential election, long ago surfaced in his own memoirs and writings. But the furor ignited by the release of the tapes by his family this month expose the lingering, unhealed wounds of the chaotic years that followed Iran's 1979 revolution. "All advice and criticism from my father was for saving the ruling system — which he had paid so much for," his son, Ahmad Montazeri, recently wrote online. In 1985, Montazeri was selected as Khomeini's successor. But Montazeri's calls for the loosening of clerical control in Iran drew the ire of hard-liners — and then came the executions. They happened at the end of Iran's long war with Iraq, which began when Saddam Hussein invaded in 1980. By 1988, 1 million people had been killed in a conflict that featured trench warfare, Iranian human-wave attacks and chemical weapons assaults launched by Iraq. In July 1988, Khomeini accepted a UN-brokered ceasefire, calling it "more deadly to me than poison." But within days, members of the Iranian opposition group Mujahedeen-e-Khalq, heavily armed by Saddam, stormed across the Iranian border in a surprise attack. Iran ultimately blunted their assault, but the attack set the stage for the sham retrials of political prisoners, militants and others that would become known as "death commissions." Some who appeared were asked to identify themselves. Those who responded "mujahedeen" were sent to their deaths, while others were questioned about their willingness to "clear minefields for the army of the country," according to a 1990 Amnesty International report . International rights groups estimate that as many as 5,000 people were executed, while the MEK puts the number at 30,000. Iran has never fully acknowledged the executions, apparently carried out on Khomeini's orders, though some argue that other top officials were effectively in charge in the months before his 1989 death. This month, a website run in Montazeri's honor by his family released the audio. In it, he apparently addresses prosecutors, a judge and an intelligence official over the executions, warning they will tarnish Khomeini's image. "I believe that the biggest crime in the history of Iran, which will be condemned by history, happened by your hands," Montazeri says. He goes on to say that "fighting against ideology with killing is totally wrong." He mentions the death of his son, Mohammad, in the 1981 bombing of the Islamic Republican Party's headquarters in Tehran, which killed at least 72 people. Iran blamed the attack on the MEK, and the US State Department also has said the group was responsible for it. "I got hurt more than anybody from them; they killed my son," Montazeri says. Shortly after the audio's release, the website took it down. By that point, however, the clip had circulated online as Farsi-language media began reporting on it, sparking criticism of Montazeri's family for releasing it. "My motivation for publishing this file, as I said for several times to many media, was to defend the truth and innocence of Ayatollah Montazeri," his son, Ahmad, recently wrote Iran executes at least 36 in August In response to the execution of 12 people on August 27 for alleged drug offenses, and 24 Kurdish prisoners earlier in the month despite a lack of due process, Freedom House issued the following statement: "Iran's staggering number of executions, including for crimes that do not meet ‘most serious crimes' under international law, should be the focus of a UN Human Rights Council inquiry," said Dokhi Fassihian, senior program manager for Middle East and North Africa programs. "For far too long, Iranian courts have taken the lives of its citizens through veiled procedures that violate the right to life in contravention of international and domestic law." Iran has been the world's highest per capita executioner for a decade. The Iranian government is one of the few that continues to execute youth under the age of 18, and political prisoners. The government claims that it is executing drug traffickers as a deterrent, though defendants are regularly denied fair trial and the right to appeal. Iran carries out executions without due safeguards, and without accountability or transparency, signaling that executions are being used as a means to deter political dissent. Since 2006, human rights groups have documented at least 5,000 executions, although the actual number of individuals put to death may be higher. Iran is rated Not Free in Freedom in the World 2016, Not Free in Freedom of the Press 2016, and Not Free in Freedom on the Net 2015.