Story highlights Some Democrats want Facebook executive to testify before congressional hearings

Facebook told investigators Wednesday Russian 'troll farms' had bought ads in 2016

(CNN) Facebook officials spent hours Wednesday briefing House and Senate staffers digging into Russia's meddling in the US election, but left investigators more frustrated than before -- demanding more answers but unsure yet of how to get them.

Facebook officials revealed that Russian "troll farms" -- likely operated by the Kremlin -- had purchased $100,000 in ads , just a few months after saying they had seen no evidence of Russian purchases. The dollar amount is small but those ads, rather than pushing a simple message, likely plugged users into a propaganda network by asking them to "Like" certain groups, one Hill source familiar with briefings said Thursday.

"This is just the tip of the iceberg," said Sen. Mark Warner, the top Democrat on the Senate intelligence committee , who has been pressing Facebook for months to cooperate with Russia investigators.

Warner noted that the ads directed users to like certain Facebook groups, which could then be used to either influence votes or, more likely he said, depress voter turnout in certain areas.

"The Russians used resource and media platforms to try to interfere with our elections, and I think the fact that they may only have mentioned a particular candidate in a few of the ads is reflective of the fact that there's ways to do voter suppression without mentioning a candidate's name," Warner said. "I think that the American people deserve to know both the content and the source of the information that is being used to try to affect their votes."

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