A nonpartisan group of election law experts is calling on Facebook to disclose the ads purchased by accounts tied to Russia during the 2016 presidential election to the American people.

In a letter to Facebook founder and CEO Mark Zuckerberg sent Tuesday, the Campaign Legal Center said allowing the public to view the content of the ads purchased by Facebook accounts linked to Russia would help shed light on how a foreign actor interfered in the election.

"Facebook appears to have been used as an accomplice in a foreign government's efforts to undermine democratic self-governance in the United States," Trevor Potter, president of the Campaign Legal Center, wrote. "Therefore, we ask you, as the head of a company that has used its platform to promote democratic engagement, to be transparent about how foreign actors used that same platform to undermine our democracy."

Facebook revealed last week it tracked $100,000 in ad spending from June 2015 to May 2017 to roughly 3,000 ads. Those ads were connected to 470 fake accounts, which Facebook traced back to a Russian "troll farm" called the Internet Research Agency.

The majority of the ads didn't mention the 2016 presidential election, Hillary Clinton, or Donald Trump, Facebook said. Instead, the ads focused on amplifying politically and socially divisive issues such as immigration, gun rights, race, and gay rights. One-quarter of the ads targeted Facebook users in specific parts of the U.S., and more of the ads purchased ran on Facebook in 2015 than in 2016.

One analysis found as few as 23 million and as many as 70 million Facebook users could have viewed the ads.

Facebook shared its findings with the Senate Intelligence Committee and Robert Mueller, the special counsel leading the Justice Department's investigation into Russian meddling in the 2016 election.

The company also showed samples of the ads tied to Russia to Senate Intelligence Committee staff, Sen. Mark Warner, D-Va., the top Democrat on the panel, said last week.

Like the Campaign Legal Center, Warner urged Facebook to share the content of the ads with the American people.

In its letter to Zuckerberg, the Campaign Legal Center reiterated federal election law prohibits foreign nationals from spending money in connection with any U.S. election, and argued the Russian nationals who created and purchased the Facebook ads broke the law by making campaign expenditures.

"Put another way, Facebook was secretly paid to host illegal political ads as part of an illegal foreign influence effort," the group said. "Yet, Facebook has refused to make the ads public. Releasing those advertisements could allow the country to better understand the nature and extent of interference with our democracy."

Facebook said last week making the ads available to the public would violate its privacy rules.