The United Nations' top chemical weapons investigator in Syria has called for a new investigation to determine who was responsible for the chemical weapons attacks that killed hundreds.
Ake Sellstrom said that if there was no accountability "I will think it's sad".
He led a team that confirmed the use of chemical weapons in a major attack near Damascus on August 21 and their probable use in four other locations against civilians and soldiers.
His team's mandate was to determine whether chemical weapons were used, but not to establish who was responsible.
Mr Sellstrom said using chemical weapons was "a hideous crime ... so it's logical that this should be followed up and brought to court somehow, or brought to a tribunal, or brought to something".
He said his team gathered "lots of facts" but not enough to determine "the guilty party in this".
To determine who used chemical weapons, a much broader investigation was needed, Mr Sellstrom said.
He said last Friday that his team did not have the freedom of a police force in carrying out its investigation.
There were "a lot of other facts with the Syrian government, with the opposition, with several capitals", he said, citing possible information on transport of chemical weapons, on militias, and on conversations that may have been overheard and recorded as well as other intelligence.
Key witnesses could also be found in Syria and at the sites of the attacks, he added.
"Someone must have given the order," Mr Sellstrom said. "There must have been consequences somewhere - and that we could be able to pick up if people are willing to give that information to (a) member state or to such an inquiry."
UN secretary general Ban Ki-moon called for accountability for the chemical weapons attacks when he addressed the world body's General Assembly and Security Council on Mr Sellstrom's findings, saying the perpetrators of "gross violations" of international human rights and humanitarian law must be brought to justice.
When pressed on Monday on how to get accountability, Mr Ban noted that the Geneva-based UN Human Rights Council was investigating alleged crimes against humanity in Syria and said he was consulting member states "on what kind of measures should be taken and when and how".
UN spokesman Martin Nesirky added that "there are a number of different methods that could be used that are available to member states, but it would be for them to do that".
A commission created by the Human Rights Council says both sides have committed heinous war crimes during the two and a half-year Syrian conflict, which has killed more than 100,000 people.
A confidential list of suspected criminals is being produced by the commission and kept under lock and key by UN human rights chief Navi Pillay. The commission could eventually decide who bears responsibility for chemical weapons attacks.
Mr Sellstrom also noted another approach the UN could take, pointing to independent UN commissions that investigated the assassinations of former Lebanese prime minister Rafik Hariri in 2005 and ex-Pakistani prime minister Benazir Bhutto in 2007.
The chief inspector said he may speculate privately about who was responsible for the attacks but he did not have clear evidence.
"I'm not surprised there are speculations on both sides," Mr Sellstrom said.
The confirmed use of chemical weapons in the Damascus suburb of Ghouta on August 21, which, according to the US government, killed 1,400 people led to a US-Russian agreement to eliminate Syria's chemical weapons by mid-2014.
The process of getting chemicals that can be used to make weapons out of Syria is currently under way.
Mr Sellstrom said inspectors now know that Syria possessed the deadly nerve agents sarin - which was used in the Ghouta attack and probably others - as well as VX. He also noted that sarin was made by the doomsday cult that carried out the 1995 Tokyo tube attack, and could be made by other groups as well.
He said he believed the Syrian government took "a strategic decision" to eliminate its chemical weapons stockpile and precursors.
"Maybe I'm naive or trying to be too positive," he said. But "I'm a believer that this strategic decision will be fulfilled."
Once the Syria conflict ends, Mr Sellstrom said, "I would love to come inside people's brains and to know what was the truth behind all this".