By Frances Harrison
BBC News, Tehran
Iran's Supreme Court has acquitted a group of men charged over a series of gruesome killings in 2002, according to lawyers for the victims' families.
The accused men believed they were allowed to kill immoral people
The vigilantes were not guilty because their victims were involved in un-Islamic activities, the court found.
The killers said they believed Islam let them spill the blood of anyone engaged in illicit activities if they issued two warnings to the victims.
The serial killings took place in 2002 in the south-eastern city of Kerman.
The case raises serious questions about vigilantes in Iran taking justice into their own hands and undermining the rule of law.
Up to 18 people were killed in just one year, but only five of the murders were tried in court.
According to their confessions, the killers put some of their victims in pits and stoned them to death. Others were suffocated. One man was even buried alive while others had their bodies dumped in the desert to be eaten by wild animals.
The accused, who were all members of an Islamic paramilitary force, told the court their understanding of the teachings of one Islamic cleric allowed them to kill immoral people if they had ignored two warnings to stop their bad behaviour.
But there was no judicial process to determine the guilt of the victims in these cases.
The group even killed a young couple they thought were involved in sex outside marriage, but media reports say the couple were either married or engaged to be married.
Lawyers for the victims' families say the Supreme Court has five times overturned the verdict of a lower court that found all the men guilty of murder.
Now the Supreme Court is reported to have acquitted all the killers of the charge of murder on the grounds that their victims were all morally corrupt.
Some of the group may, however, face prison sentences or have to pay financial compensation to their victims' families.