A former Salvadorean army colonel has been be extradited to Spain to stand trial over the killing of six Jesuit priests in El Salvador in 1989.
Col Inocente Orlando Montano, 76, was flown from the US, where he had been living, to the Spanish capital, Madrid, where he was taken to prison.
The killings caused outrage not just in El Salvador but also in Spain, where five of the murdered priests were from.
Col Montano has denied any wrongdoing. He is to be questioned on Thursday.
The judge in the case will then decide whether to keep him in pre-trial detention.
The five Spanish-born Jesuit priests were shot dead along with a Salvadorean priest, their housekeeper and her 16-year-old daughter on 16 November 1989 by an elite unit of the Salvadorean army.
Col Montano was the deputy minister of public security at the time of the killing. Prosecutors allege he helped plot it and provided the killers with key information.
The security forces suspected the priests, who worked at the Central American University, of sympathising with left-wing rebels of the Farabundo Marti National Liberation Front (FMLN).
Brutal civil conflict
According to a 1993 UN Truth Commission report, more than 75,000 people were killed during the 12-year civil war, which pitted the left-wing rebels against the army and right-wing death squads.
A UN-brokered peace process brought the conflict to an end in 1992, but a 1993 amnesty law meant that civil-war era human rights violations went unpunished.
Col Montano had been living in the US since 2001 and was working in a sweet factory in the US state of Massachusetts when he was arrested on immigration charges in 2011.
He was sentenced to nearly two years for immigration fraud and perjury.
While he was in prison, a Spanish judge requested his extradition to face charges under Spain’s universal jurisdiction law.
The law holds that some crimes are so grave that, even when they have not been committed on Spanish territory, they can be tried there.