San Francisco and Washington DC (CNN) An explosive claim by the US State Department last weekend that there are thousands of Russian-linked social media accounts on Facebook and Twitter spreading disinformation about the coronavirus made headlines around the world.

But almost a week later, Facebook and Twitter say the State Department has yet to provide evidence that would allow the companies to investigate and possibly shut down the accounts.

"We would love to get a briefing on this," Yoel Roth, who leads Twitter's team investigating coordinated influence campaigns, said at the RSA cybersecurity conference in San Francisco Thursday.

Speaking at the same event, Nathaniel Gleicher, Facebook's head of cybersecurity, said, "We have asked for any evidence that supports this, we haven't received anything yet."

The lingering questions expressed publicly by senior staff at these tech companies may raise new doubts about how the US government is working with Silicon Valley to combat Russian disinformation targeting Americans as well as how the government determines where online disinformation is coming from.

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