* Investigators obtain trove of official Syrian documents
* Seeking to build prosecution "along lines of Nuremberg"
* Documentary on torture of Syrian detainees has premiere
By Stephanie Nebehay
GENEVA, March 15 (Reuters) - Criminal investigators say they have built a case documenting the widespread torture and murder of Syrian detainees by the Assad government, relying on official photos and meticulous documents.
More than 700,000 pages from Syrian intelligence and security archives have been smuggled out by the Commission for International Justice and Accountability (CIJA), an independent group of legal experts, through a secret network.
"The documentation is, in the main, generated by the security-intelligence, military and political structures of the regime," William Wiley, who has worked for U.N. war crimes tribunals on former Yugoslavia and Rwanda, told Reuters.
A key document from 2011 orders the arrest of protesters or people in contact with foreign media, he said in a new documentary "Syria's Disappeared: The Case Against Assad" that tracks Wiley and his group's work in Syria.
In another, an official asks what to do with a "hospital refrigerator full of unidentified corpses that have decomposed".
"This person copied the Ministry of Justice, so a localised problem is being brought to the attention of the regime," said Wiley, executive director of CIJA, a non-profit foundation that is also preparing cases in Iraq for prosecution.
The government of President Bashar al-Assad denies findings by U.N. investigators that detainees are tortured and executed in a policy of "extermination", in the war that is entering its seventh year.
"The queen and king of evidence in any criminal investigation is a document. It isn't cross-examined because it is factual, it is truth," Wiley said in the documentary.
The film, which had its premiere at the International Film Festival and Forum on Human Rights in Geneva this week, includes interviews with former detainees and grieving mothers.
"We are trying to lay the foundation for prosecution along the lines of Nuremberg," Wiley said in the documentary.
His group is funded by countries including Britain, Canada and Germany.
Stephen Rapp, former U.S. ambassador for war crimes issues, said of Syrian authorities: "They were keeping meticulous records."
"This is the legal equivalent of a 'slam dunk'," he said in the film.
Investigators have access to 55,000 photos of detainees' bodies - some with gouged out eyes - smuggled out by a former forensic photographer code-named Caesar who worked at Tishreen military hospital.
All files have been moved to a secret location in Europe with the aim of sharing them with national judicial authorities or a future international court.
"The real potential (for prosecution) is with national war crimes units domestically," Wiley told the film's audience.
Lawyer Almudena Bernabeu, shown in the film, has lodged a case in Spain, alleging "state terrorism" on behalf of a Syrian-Spanish woman who recognised her dead brother among the Caesar photos. A judge will rule on whether the case is admissible.
At the screening, former detainee and activist Mazen Al Hamada, who was released, says interrogators broke his ribs, hung him by handcuffed wrists, and sexually assaulted him.
"We want those perpetrators to be tried, we have to be patient," he said to a standing ovation. (Reporting by Stephanie Nebehay; Editing by Alison Williams)