Laboratory tests have for the first time linked a Syrian government chemical weapons stockpile to the largest sarin nerve gas attack of the country's seven-year civil war.
Diplomats and scientists confirmed the link to Reuters. The proof of the link supports the original Western claims that Syrian President Bashar Al-Assad was behind the attack.
The sarin was used in the early hours of 21 August, 2013, in the Damascus suburb of Ghouta. Estimates of how many people died range from 281 to 1,729, with around 3,600 people injured.
Sarin gas has no taste or smell. Estimated to be 26 times more deadly than cyanide, it can cause paralysis, convulsions and death in as little as 10 minutes.
A UN mission took samples from the attack site, which were then compared to chemicals handed over by the Syrian government for destruction in 2014. The tests were carried out by laboratories working for the Organisation for the Prohibition of Chemical Weapons (OPCW).
Two people involved in the testing process said that chemical markers identified in the Ghouta samples were also found in samples from gas attacks in Khan Sheikhoun in Idlib governorate on 4 April, 2017 and Khan al-Assal, Aleppo, in March 2013, Reuters reported.
In October 2017, the OPCW-UN joint investigation used these test results to assert that the Syrian regime was behind the Khan Sheikhoun attack.
The Ghouta test results were confirmed by two separate diplomatic sources, said Reuters.
Two compounds in the Ghouta and Khan Sheikhoun samples matched. One was formed from sarin and the stabiliser hexamine; the other was a specific fluorophosphate that appears during sarin production.
Non-proliferation expert Amy Smithson said that the results were the "equivalent of DNA evidence: definitive proof".
Smithson noted that the hexamine finding was particularly significant as this chemical is a unique hallmark of the Syrian military's sarin production process.
Other evidence, including the quantity of sarin used - hundreds of litres - and radar images of rocket traces leading to Syrian military positions, also pointed to the Assad regime.
"This match adds to the mountain of physical evidence that points conclusively to the Syrian government," Smithson said.
A US-Russian deal in the aftermath of the Ghouta attack agreed that Damascus would hand over its chemical weapons stockpiles for destruction. The weaponised use of chlorine and sarin continued, however.
This Khan Sheikhoun attack prompted US President Donald Trump to order a missile strike against the Shayrat air base, from where the attack was believed to have been launched.