French intelligence services have concluded that forces loyal to Syrian President Bashar al-Assad carried out a sarin nerve gas attack on 4 April in northern Syria.
They also said that Mr Assad or his closest entourage ordered the strike, a declassified report showed.
The attack in the town of Khan Sheikhoun killed scores of people and prompted the United States to launch a cruise missile strike on a Syrian air base in response, its first direct assault on the Assad government in the conflict.
The six-page document - drawn up by France's military and foreign intelligence services and seen by Reuters - said it was able to reach its conclusion based on samples they had obtained from the impact strike on the ground, and a blood sample from a victim.
Among the elements found in the samples were hexamine, a hallmark of sarin produced by the Syrian government.
"The French intelligence services consider that only Bashar al-Assad and some of his most influential entourage can give the order to use chemical weapons," the report said.
It added that jihadist groups in the area did not have the capacity to develop and launch such an attack and that the so-called Islamic State was not in the region.
Mr Assad's claim to AFP news agency on 13 April that the attack was fabricated, was "not credible" given the mass flows of casualties in a short space of time arriving in Syrian and Turkish hospitals as well as the sheer quantity of online activity showing people with neurotoxic symptoms, said the report.
French Foreign Minister Jean-Marc Ayrault said: "We know, from a certain source, that the process of fabrication of the samples taken is typical of the method developed in Syrian laboratories.
"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."