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The FBI monitored social media on Election Day last year in an effort to track a suspected Russian disinformation campaign utilizing "fake news," CNN has learned.

In the months leading up to Election Day, Twitter and Facebook were the feeding grounds for viral "news" stories floating conspiracies and hoaxes, many aimed at spreading negative false claims about Hillary Clinton.On Election Day, dozens of agents and analysts huddled at a command center arrayed with large monitoring screens at the FBI headquarters in Washington watching for security threats, according to multiple sources.

Read alsoRussia collusion probe entering new phase in U.S. as grand jury impaneledThat included analysts monitoring cyber threats, after months of mounting Russian intrusions targeting every part of the US political system, from political parties to policy think-tanks to state election systems.

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On this day, there was also a group of FBI cyber and counterintelligence analysts and investigators watching social media.

FBI analysts had identified social media user accounts behind stories, some based overseas, and the suspicion was that at least some were part of a Russian disinformation campaign, according to two sources familiar with the investigation.