Facebook CEO Mark Zuckerberg and his company have declined to disclose the ads purchased last year by accounts linked to Russia. Reuters Pictures/Carlos Garcia Rawlins

Despite growing pressure on the company, Facebook says that it's "unable" to publicly reveal which ads were purchased on its platform by accounts linked to Russia during the 2016 US presidential election.

"Due to both federal law and the fact that investigations are ongoing with the relevant authorities, we’re unable to share the ads," a Facebook spokesperson told Business Insider on Friday. The spokesperon declined to say which legislation kept the company from disclosing the ads.

Earlier this week, the social network revealed that a network of fake accounts linked to a known Russian organization that runs an online influence operation purchased a total of approximately $100,000 in ads intended to amplify divisive political messages.

Since then, pressure has been mounting on Facebook to publicly reveal the ads themselves. On Thursday, Senator Mark Warner, the vice chairman of the Senate Intelligence Committee, called on Facebook to reveal to the public the ads it said were linked to Russian actors.

Americans "ought to be able to know if there is foreign-sponsored [internet] content coming into their electoral process," Warner said Thursday during a panel discussion at an event hosted by the Intelligence and National Security Alliance. "That becomes a method of influence exponentially, I would argue, bigger than TV and radio."

In a blog post announcing the Russian ads on Wednesday, Facebook chief security officer Alex Stamos said the company had shared its "findings with US authorities investigating these issues." According to Reuters, Facebook gave its findings to US special counsel Robert Mueller, who is leading the government's probe into Russian interference with last year’s election.

When asked by Business Insider on Friday, a Facebook spokesperson declined to confirm whether the company had shown the ads themselves to Mueller or shared data about how the ads were specifically used to target users of its site. The spokesperson also declined to say whether Facebook would reveal the ads publicly after Mueller's investigation was concluded.

Some experts estimate the ads could have reached and influenced more than 30% of the US population.

'Pretty crazy'

President Donald Trump allegedly benefitted from the ads. Shawn Thew-Pool/Getty Images

Facebook's revelation about the Russian ads followed months of denials by the company that Russia or other organized actors used its platform to influence election results through the dissemination of fake news.

Days after Donald Trump won the election, for example, Facebook CEO Mark Zuckerberg dismissed the idea that his platform had been used to manipulate voters as "pretty crazy." And in July, Facebook told CNN that it had found "no evidence that Russian actors bought ads on Facebook in connection with the election."

The roughly $100,000 in ads that Facebook has now said were linked to Russia appear to originate from The Internet Research Agency, a shadowy, Russia-backed professional online influence operation that was dubbed an "army of well-paid trolls" in lengthy exposé published in The New York Times Magazine in June 2015.

"Public reports in 2015 described a troll farm called the Internet Research Agency," a Facebook spokesperson told BI on Friday. "Our research links what we've announced to that group."

It's currently unclear if the ads themselves, regardless of their origin, violated federal law, which prohibits foreign nationals from spending money to influence a US election.

Facebook on Wednesday said the "vast majority" of the ads related to the fake Russian accounts didn't target a political candidate and instead focused on divisive topics like LGBT issues, immigration, and gun rights.

The social-networking giant has historically declined to reveal details about political ad spending on its network. But Russia's spending around last year's election has prompted discussion about whether online ads should be subject to the same disclaimers that are required for ads on TV and other types of media.

"An American can still figure out what the content is being used in TV advertising. You can go look at the ad," Senator Warner said on Thursday. "But in social media there is no such requirement. So, you know, we may need a legislative solution."