At least one Facebook employee that worked with President Trump’s presidential campaign has been interviewed by special counsel Robert Mueller’s team as part of the investigation into Russian meddling in the 2016 election, according to a report.

Facebook sent staff to work with members of the Trump campaign’s digital team, and a source told Wired that Mueller’s team of investigators have sat down with at least one person who was dispatched to assist Trump.

In addition to Facebook, Twitter and Google sent employees to assist with the Trump campaign’s digital operations, Wired reported. All three social media companies also helped Hillary Clinton’s campaign.

Facebook has been criticized for its role in Russia’s efforts to interfere in the election. The company disclosed over the summer that it sold thousands of ads to accounts linked to a Russian troll farm called the Internet Research Agency.

Several congressional committees investigating Russian meddling have honed in on Facebook, Twitter, and Google for housing content backed by Russia.

Representatives from the three companies testified before the Senate Intelligence Committee at a November hearing, and they have been working to determine how many users were exposed to content tied to Russian influence operations.

Colin Stretch, Facebook's general counsel, told the Senate Intelligence Committee last year that nearly 150 million Facebook and Instagram users were exposed to Russia-backed content.

Twitter said last week that 677,775 people interacted with content from Russian accounts. The company said it will notify those users.

Google told lawmakers last year 18 YouTube channels with roughly 1,100 videos were connected to the Internet Research Agency, and those accounts spent $4,700 on ads.