Transcript for Propaganda casting doubt on chemical weapons attack gained traction in US

This comes one week after the U.S. Missile strike on Syria. That action was taken after the chemical attack killed so many, including children. Now Syrian president is fiercely denying that he launched that attack on his own people. Asking, where those children even dead at all? Here's chief investigative correspondent Brian Ross. Reporter: Officials in Syria tonight are claiming these horrific videos of last week's chemical attack were staged, that the children on the edge of death, the grieving father with his twins, were all just part of a hoax. U.S. Officials say there is solid forensic and electronic evidence to prove it was real and that Syria was to blame. Were they dead at all? Reporter: Today the Syrian president, bashar Al Assad said it was just a pretext for the U.S. Missile strike. Definitely, 100% for us, it's fabrication. We don't have an arsenal. We're not going to use it. Reporter: U.S. Analysts believe today's pushback is part of a shameless Russian fake news operation that began even before last week's U.S. Strike. That night, a cyber-operation with the #syriahoax, pushed by the Russians, became the number one trending topic on Twitter in the U.S. It was picked up by Russian influencers online who pushed the story and the hashtag until it entered the alt-right community in the United States. Reporter: The disinformation is believed to be coming from the same secretive Russian intelligence operation that last year targeted the American presidential election. We know that they have a large building in St. Petersburg that is the home base for a team that is tasked with manipulating public opinion in other countries. And at this secretive British spy listening headquarters outside London, where ABC news was given rare access, analysts say the Russian disinformation operation has become ever more aggressive since the American election. It has not plateaued. It is continuing to increase. It is a very capable cyberattack operation. Brian breaking news on another front this evening. . The British newspaper the guardian is out with an eye-opening headline, they're reporting tonight that British intelligence services intercepted conversations between known Russian spies and people working in the trump campaign and Brian, you confirmed that conversations were in fact picked up. Yes, David a senior intelligence officials confirms to ABC news the British did pick up those conversations as part of routine surveillance of Russian spies. Those conversations happened in the early 2015. We don't foe who in the campaign was heard talking. Brian, thank you. We turn to another developing headline from

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