French intelligence has identified the chemical “signature” of the Syrian government at the site of a nerve agent attack this month, indicating that the sarin used in the bombing came from Bashar al-Assad’s stockpiles.

Samples from the scene of the attack in the town of Khan Sheikhun, contained chemical compounds that were a hallmark of Assad’s sarin manufacturing process and matched samples collected from the site of a prior attack by government forces, a declassified intelligence report says.

It also indicates that the sarin used in the 4 April attack, which killed more than 80 people, was delivered from the air, adding further weight to the international consensus that the Assad regime was responsible for the massacre.

“Based on this overall evaluation and on reliable and consistent intelligence collected by our services, France assesses that the Syrian armed forces and security services perpetrated a chemical attack using sarin against civilians,” the report says.

The Khan Sheikhun attack drew international condemnation and provoked a US missile strike on a Syrian airbase.

Why would Assad have used chemical weapons? Assad’s military gains since 2015 have been slow and costly. His army is battle-weary and although it has won decisive battles, the war is far from over. Chemical weapons have therefore remained an attractive option when military alternatives run low. Moreover, the regime has repeatedly broken international laws protecting non-combatants: civilians have been repeatedly bombed and starved for years in an attempt to demoralise those in rebel-held areas. Assad's wager was that Russian support and western fear of intervention would protect him from anything more damaging than censure and outrage. Turning to sarin on 4 April was an escalation in brutality, but Assad may have felt more confident after the US said it was no longer focused on his removal.

The French intelligence report, presented by the foreign minister, Jean-Marc Ayrault, adds weight to a mounting body of evidence pointing to the Assad regime’s responsibility for the chemical attack.

The US government had reached a similar conclusion based on its own intelligence assessment and signals intelligence gathered from Syria. Tests conducted in Turkey on victims of the attack found isopropyl methylphosphonic acid, a chemical that sarin degrades into, in blood and urine samples, but the French analysis went further.

In addition to proving the presence of sarin in environmental samples collected from the impact zone in Khan Sheikhun, the French analysis indicated that the sarin was delivered using aerial munitions, which rules out the theory that the chemical attack was perpetrated by rebel forces, who do not possess an air force.



French analysis of the chemical compounds from the site also pointed to the presence of sarin, hexamine and diisopropyl methylphosphonate, a compound formed when sarin is synthesised. The chemicals found there essentially match the biological fingerprint of the Assad government’s sarin manufacturing process, as the same chemicals were found in samples collected from a 2013 attack in the town of Saraqeb in Idlib, which was also linked to the regime.

“We know, from a certain source, that the process of fabrication of the samples taken is typical of the method developed in Syrian laboratories,” Ayrault told reporters after presenting the findings to the French cabinet. “This method is the signature of the regime and it is what enables us to establish the responsibility of the attack. We know because we kept samples from previous attacks that we were able to use for comparison.”

The conclusions of the French report further undermine allegations by Moscow, Assad’s primary backer, and the Syrian government, which vacillated from blaming the attack on the rebels to arguing that it never happened, and then blaming it on an airstrike on a warehouse in Khan Sheikhun where the rebels had allegedly stockpiled sarin.

The Guardian visited the site of the massacre two days after the chemical attack and independently confirmed that the warehouse and nearby grain silos were empty and had not been bombed recently, contrary to Russian claims.

The Organisation for the Prohibition of Chemical Weapons said on 19 April that sarin or a similar banned toxin had been used in the Khan Sheikhun attack, but it is not mandated to assign blame.

The OPCW had overseen the supposed dismantling of the Syrian chemical weapons programme, a signature achievement by the previous US administration under Barack Obama. Since then, evidence has emerged that the Assad government had failed to account for its entire chemical arsenal and has instead maintained the capacity to weaponise sarin.

Since 2013, numerous incidents of chlorine gas use by the government have been reported, with limited casualties. The Khan Sheikhun attack was the largest mass casualty incident involving chemical weapons since the Ghouta massacre of 2013, widely blamed on Assad’s forces, which killed more than 1,400 people in opposition territory near Damascus.