New evidence has been released that up to 7,000 Syrians who died in government detention centres in Syria were tortured to death, a rights group said in a report.
Human Rights Watch released a report after an eight month research of at least 53,275 smuggled photographs of tortured Syrian individuals.
In January 2014, a defector, who was given the codename of "Caesar", smuggled images out of Syria, showing bodies of detainees photographed by his colleagues and himself.
Caesar's role in the military police in Syria was to photograph and document the bodies of those brought from their places of detention to a military hospital.
Read more: Mustard gas used in Syria fighting: UN watchdog
The bodies in his images showed signs of starvation, brutal beatings, strangulation and other forms of torture and killing.
In his interview with the investigators' legal team conducted over the course of three days in January 2014, Caesar told the team he had worked in the Syrian military police for 13 years.
Most of the 6,786 victims shown in the Caesar photographs were detained by just five intelligence agency branches in Damascus.
Two military hospitals in Damascus received the bodies between May 2011, when Caesar began copying files and smuggling them out of his workplace, and August 2013, when he fled Syria.
The 86-page report, "If the Dead Could Speak: Mass Deaths and Torture in Syria’s Detention Facilities," answers questions such as: are the photographs authentic? Are they really images of dead detainees? If so, what caused so many to die? and how did the bodies end up in military hospitals and what happened to the corpses afterwards?
During the course of their research, HRW located and interviewed 33 relatives and friends of 27 victims whose cases researchers verified; 37 former detainees who saw people die in detention; and four defectors who worked in Syrian government detention centers or the military hospitals where most of the photographs were taken.
Dead bodies kept in cells for days
Khaled, who was released from a prison in Damascus suburbs in February, told Al Jazeera that he was held for two years after defecting from the army.
"They electrocuted us several times a week, as they asked us questions on the whereabouts of defected colleagues. I lost 13kg in less than three months, there was no food or water for days.
"We were all cramped in small cells, sometimes after torturing us they would throw us back in the cell naked. When people in the cell bled or starved to death, they would keep their bodies there for a few days to terrorise us," Khaled said.
"We could not sleep either, they would make sure we would stay up - cramped in a small cell with dead bodies around us. It haunts me until this day."
Khaled is now a refugee in Turkey, after his release he managed to escape to Lebanon and made his journey to Adana.
Read more: Children tortured and used as child soldiers in Syria's war
Using satellite imagery and geolocation techniques, HRW confirmed that some of the photographs of the dead were taken in the courtyard of the 601 Military Hospital in Mezze, a neighbourhood in Damascus.
Detainees told HRW they were provided with such insufficient food that they slowly starved; one former detainee told the group he lost 35 kilos, nearly half his body weight, in just six months of detention.
In a January 2015 interview with Foreign Affairs, Syrian President Bashar al-Assad responded to allegations that prisoners have been tortured and abused by Syria, saying: "If there’s any unbiased and fair way to verify all those allegations, of course we are ready. That would be in our interest."
The Syrian Network for Human Rights, a group monitoring the war, documented the arrest and detention of more than 117,000 people in Syria since March 2011.
SNHR documented at least 11,358 deaths in detention as of June 26, 2015. The total number is likely higher given that many cases go unreported.
Source: Al Jazeera