A new report from three international war crimes prosecutors says the Syrian government committed the “systematic killing” of detainees under their control.

The report, which was leaked to the Guardian newspaper, shows that 11,000 detainees between March 2011 and August 2013 died at the hands of government jailers and military officials.

The source was a military police photographer who defected from the country in 2013.  Before leaving the country, he collected memory sticks that contained thousands of photos of detainees killed while in custody.

The defector who is identified only as “Caesar” told investigators that he photographed as many as 50 bodies a day.  He said the purpose of the photos was to allow a death certificate to be produced while denying their families access to the body.  The cause of death was usually listed as a heart attack or “breathing problems.”

The report was commissioned by a London law firm on behalf of the government of Qatar which has claimed Syrian President Bashir al-Assad has committed multiple war crimes during the country’s current civil war.