In those briefings, Facebook spoke only in generalities about the ad buys, leaving some committee members feeling frustrated with Facebook's level of cooperation. Facebook's policy states that, in accordance with the federal Stored Communications Act, it can only turn over the stored contents of an account in response to a search warrant.
Facebook, the world's largest social network, said last week that an operation likely based in Russian Federation had placed thousands of US ads with polarizing views on topics such as immigration, race and gay rights on the site during a two-year period through May 2017.
The information is relevant to Mueller as investigators try to understand whether there were any links between Russia's activity and President Donald Trump's election campaign. Facebook marketing is becoming increasingly important for election strategy, but social media ads are not legally required to provide the same transparency as ads that run on more traditional forums like television. CNN subsequently reported a warrant was used in obtaining the information.
"I think there's much more than 350-odd accounts that were involved in this process", Warner said, referring to ads linked to Russian Federation that the company acknowledged last week.
Rangappa explained that to obtain a search warrant a prosecutor needs to prove to a judge that there is reason to believe a crime has been committed.
Some experts however pointed out that the new direction in the case also means that Mueller is no longer looking at Russia's election interference from a strict counterintelligence standpoint.
"That means that he has uncovered a great deal of evidence through other avenues of Russian election interference", she said.
Former federal prosecutor Renato Mariotti echoed Rangappa's statement.
"Platforms like Facebook and Twitter really need to put technical measures in place that would prevent the spread of fake information or the creation of groups that may promote hate speech", Gruzd said. The Senate Intelligence Committee is likely to ask Facebook representatives to testify publicly in the coming weeks, but even if it subpoenas Facebook, it is not likely to gain access to the information Mueller now has at his fingertips.
Further, the House Intelligence Committee is seeking an interview with Brad Parscale, the digital director for Trump's campaign, who worked closely with Trump's son-in-law Jared Kushner.