STOCKHOLM (Reuters) - A 58-year-old Iranian citizen has been arrested in Sweden on suspicion of murder in Iran in 1988, prosecutors said on Wednesday, and national news agency TT said the case involved killings of political prisoners.

Human rights groups say the Islamic Republic executed thousands of political detainees in the late 1980s. Many of them belonged to opposition groups. Iran denies having political prisoners or having executed any such prisoner.

The prosecutor’s detention request said the man was arrested on Nov. 9 after what TT said was his arrival to visit relatives. He was remanded in custody Wednesday pending a decision by Dec. 11 on whether he should be formally charged and put on trial.

TT said the man was suspected of playing a leading role in a prison in Karaj, a city west of the Iranian capital Tehran, where many detainees were executed in 1988.

The man’s lawyer was not immediately available for comment. The lawyer told TT the man denies the allegations.

Iranian officials have denied the late 1980s killings took place. However, in 2016 a 28-year-old audio tape surfaced in which Ayatollah Hossein Ali Montazeri, the deputy Supreme Leader of Iran at the time, said the executions were the “biggest crimes committed by the Islamic Republic”.

Amnesty International said in a report last year that the lowest estimates put the number of executed at around 5,000.