Communications featuring Syrian military and chemical experts discussing preparations for the Sarin attack in Idlib last week were intercepted by the US military and intelligence community, a senior US official told CNN news network on Thursday.
The intercepts were part of an immediate review of all intelligence in the hours after the attack in northwestern Syria, which killed at least 74 people and injured more than 550, according to the Idlib health authority.
After studying the intercepted communications, US officials said that there is "no doubt" that Syrian President Bashar al-Assad is responsible for the attack.
The official emphasized that the US did not know prior to the attack that it was going to happen, stating that the US gathers such a massive volume of communications intercepts in areas like Syria and Iraq, that the material is rarely processed unless there is a particular event that requires analysts to go back and search for supporting intelligence material.
So far there are no intelligence intercepts that have been found directly confirming that Russian military or intelligence officials communicated about the attack.
The official said that the Russians are more cautious in their communications to avoid being intercepted, and if those did take place they were likely to have been careful enough as to not leave an obvious record or trace.
The Russian and Syrian governments have both denied involvement in the chemical attack.
President Donald Trump told a White House news conference Wednesday that the Pentagon is looking into the question of Russian complicity in the chemical attack.
"I would like to think that they didn't know, but certainly they could have," Trump said. "(Defense Secretary) Gen. (James) Mattis is looking into it with the entire Pentagon group that does that kind of work."
The US now believes that Syria has re-established a unit of personnel associated with chemical weapons that existed before the 2013 agreement in which the Syrian government pledged to give up its weapons inventory. There is also some indication that they are receiving outside help.
"We know they have the expertise. And we suspect that they have help," a US military official told reporters at a background briefing Friday.
At the same briefing, the official also noted: "We know the Russians have chemical expertise in-country. We cannot talk openly about any complicity between the Russians and the Syrian regime in this...in this case, but we're carefully assessing any information that would implicate the Russians knew or assisted with the Syrian capability."