France's foreign minister says the proof is in the chemical makeup. The April 4 attack killed dozens of people, and the gas used matched samples from a 2013 chemical attack that killed more than 1,400.
The minister said: "We have definite sources that the procedure used to make the sarin [gas] sampled is typical of the methods developed in Syrian laboratories. This method bears the signature of the regime."
The 2013 attack was connected to the Syrian government using U.S. intelligence recordings, which picked up the entire process, from preparations to assessing the aftermath.
An official told CNN that U.S. intelligence agencies again intercepted communications between Syrian officials and chemical experts before the April 4 attack.