A Spanish judge has summoned three Iraqi officers over a raid by Iraqi security forces on a camp housing an Iranian exile group.
The UN says 34 people were killed in the raid at Camp Ashraf, in Diyala province, in April 2011.
Judge Fernando Andreu has summoned Gen Ali Ghaidan Majid, the head of army, and two other officers to appear.
He is investigating allegations that crimes against humanity were committed during the raid on the camp.
The investigation is an enlargement of an existing probe on a separate raid which took place at the camp in July 2009, in which 11 people were killed.
Under Spain's universal justice doctrine, grave crimes committed in other countries can be prosecuted.
Judge Andreu said that the Geneva Convention applied to the case, as it addresses the protection of civilians in wartime and all those killed and injured in the attack were considered "protected persons" under the terms of the Convention.
According to documents released by Madrid's investigative court, a total of 377 "protected persons" were injured in the 8 April 2011 raid, 154 with bullet wounds.
More than 3,000 members of the banned opposition group, the People's Mujahideen of Iran (PMOI), have been confined by the US military at the camp since the invasion in 2003.
The group, considered a terrorist group by the US and Iran, were given permission to shelter in Iraq by former President Saddam Hussein during the 1980-88 war between the two countries and they have lived at the camp ever since.
In January, the judge had said he would close the dossier into the July 2009 attack if the Iraqi authorities opened their own investigation.
Iraq responded by saying it had carried out its own legal inquiry but this was not judged sufficient by Spanish authorities.
The three Iraqi officers have been summoned to appear before the Madrid court on 3 October 2011.