(CNN) The US military and intelligence community has intercepted communications featuring Syrian military and chemical experts talking about preparations for the sarin attack in Idlib last week, a senior US official tells CNN.

The intercepts were part of an immediate review of all intelligence in the hours after the attack to confirm responsibility for the use of chemical weapons in an attack in northwestern Syria, which killed at least 89 people. US officials have said that there is "no doubt" that Syrian President Bashar al-Assad is responsible for the attack.

The US did not know prior to the attack it was going to happen, the official emphasized. The US scoops up such a large volume of communications intercepts in areas like Syria and Iraq, the material often is not processed unless there is a particular event that requires analysts to go back and look 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 the likelihood is the Russians are more careful in their communications to avoid being intercepted.

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 assesses 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. And there is some indication they are getting 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 that briefing, the official also noted, "We know the Russians have chemical expertise in-country. We cannot talk about openly 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."

But even if there is a definitive finding of Russian complicity, it's not clear the Pentagon or the White House would make that information public, a senior US official said.

First, it would have to be ironclad proof, which could be difficult to determine.

But also, the US feels right now that it has made the case that Russian support for Assad must end.

For now, the official said the most specific evidence of Russian involvement remains a Russian drone that flew over the hospital that was treating people injured in the attack. The US has specific intelligence showing it was a Russian drone. While the drone operator may not have known why the aerial vehicle was flying in the area, it was a Russian-controlled asset.