To draw its conclusions over the battle for Aleppo, the United Nations relied on video footage and Skype interviews because its investigators are banned from entering Syria.
Inquiry chairman Paulo Pinheiro explains how it worked.
"Basically by Skype, by mobiles, and by videos sent to us, forensic analysis, satellite imagery. Then we use all these avenues of information, and then I think that we are able to get a very accurate vision of what was really happening from July to December, to the beginning of this year."
Mr Pinheiro says daily air strikes on rebel-controlled eastern Aleppo with the deliberate targeting of hospitals caused the deaths of hundreds of people.
"The Syrian and Russian air force relentlessly bombarded eastern Aleppo city as part of a strategy to force surrender. Hospitals, orphanages, markets, schools and homes were all but obliterated. Unable to leave after government forces laid siege to eastern Aleppo in July, hundreds of civilians, many of them children, lost their lives to the daily bombardments that used concrete piercing bombs plus ammunitions, incendiary weapons and crudely weaponised chlorine canisters on civilian-inhabited areas."
The investigators say there were an alarming number of chlorine attacks.
The use of chorine as a weapon is banned under international law as well as the Chemical Weapons Convention, to which Syria is a party.
Mr Pinheiro says the investigation found a clear picture.
"Chlorine that was launched with the support of helicopters. And, in the report, we are able to attribute several events of the use of chlorine to the Syrian forces, only to the Syrian forces. We don't have any doubt that the Russian forces were not involved in these events using chlorine."
The investigators say the evacuation of the rebel enclave in eastern Aleppo in December, which ended the battle, amounted to forced displacement.
The report has been released as peace talks continue in Geneva under the direction of UN mediator Staffan de Mistura.
Russian foreign minister Sergei Lavrov says it is vital the talks focus on reaching what he calls a unified view of how Syria will be governed during the transition period.
And he says they also must focus on coming to an agreement about the make-up of a national unity government.
Mr Lavrov stresses the importance of differentiating between what he calls moderate opposition groups from those he says chose the path of terrorism and extremism.
"But in the same resolution, there is a paragraph which is extremely important to follow. It is a demand to carry out an uncompromising fight against terrorism and to separate moderate opposition from those who chose the path of terrorism and extremism. Because it will be very difficult to achieve everything else before we realise who is a legitimate participant in the political process. It is inadmissible to allow extremist elements representing organisations banned by the UN Security Council to infiltrate this political process."
