The Campaign Legal Center is urging Facebook to release the content of political ads purchased by Russian nationals and displayed on Facebook, most notably during the 2016 presidential campaign cycle.

CNBC obtained an advance copy of a letter sent Tuesday to Facebook CEO Mark Zuckerberg from CLC President Trevor Potter. The nonprofit watchdog group accuses Facebook of having been “used as an accomplice in a foreign government’s effort to undermine democratic self governance in the United States.”

The letter urges Zuckerberg to release the ads and “allow the country to better understand the nature and extent of foreign interference with our democracy.”

Facebook recently announced that more than 3,000 ads were purchased by approximately 470 fake users between 2015 and May of this year. The company has so far refused to release the content of the ads, which it said, “appeared to focus on amplifying divisive social and political messages across the ideological spectrum — touching on topics from LGBT matters to race issues to immigration to gun rights.”

Facebook did not immediately respond to a request for comment from CNBC.

The letter marks the latest sign of growing pressure Facbeook faces from Washington, where members of Congress are demanding more information into how the Russian bots managed to purchase the ads and what Facebook will do in the future to prevent foreign influence campaigns.

One congressional critic is the ranking member of the powerful Senate Intelligence Committee, Sen. Mark Warner, D-Va., who recently said he believes the Facebook announcement was merely “the tip of the iceberg.” Speaking at a security conference, Warner also signaled that he expects more transparency from companies like Facebook, Twitter and Google about who is purchasing ads.

The Campaign Legal Center is a nonprofit watchdog group in Washington, D.C., that monitors adherence to campaign finance laws and disclosure requirements. It recently hired Walter Shaub, a former director of the Office of Government Ethics who resigned in July after repeatedly clashing with the Trump administration.

Read the entire letter, below.