The authorities in El Salvador have refused to re-open investigations into the rape and murder of three American nuns and a lay missionary eighteen years ago, despite evidence that those who ordered the attack are still free.

The chief prosecutor said it was legally impossible to revive the case, and it was now closed.

The BBC Central America correspondent says the crime was one of the most shocking episodes in El Salvador's civil conflict, and prompted the US government to cut off military aid.

Five soldiers were convicted of the killings which the government insists was a common crime.

But earlier this year, four of the men said they were following orders.