Facebook reps told congressional investigators Wednesday that it had unwittingly sold ads during the presidential election to a Russian company that was targeting voters, a new report said Wednesday.

The officials said they traced the ad sales worth $100,000 to a shadowy Russian “troll farm” with a history of pushing pro-Kremlin propaganda, the Washington Post reported, citing sources.

Some of the 3,300 ads, which first appeared in the summer of 2015, named eventual nominees Donald Trump and Hillary Clinton, the paper said.

Most of the ads — which the company would not release — focused on polarizing issues such as gun rights, immigration fears, gay rights and racial discrimination.

Facebook came clean as congressional investigators and special counsel Robert Mueller are probing Russian meddling in the US election and whether there was collusion with the Trump campaign.

The president has denied the allegations of potential collusion, dismissing them as “fake news” and “a hoax.”

US intelligence officials said in January that the Kremlin had interfered in the election to help elect Trump, including by using paid social media trolls to spread fake news intended to influence the public.

The ad spending from the Russian outfit was dwarfed by overall spending — but investigators will now focus on whether Russia was following orders from someone inside the US, the paper said.

Facebook learned about the Russian connection as part of buyers of politically motivated ads.

The company found that 3,300 ads had digital footprints that led to the Russian firm.

A Facebook official told the paper that “there is evidence that some of the accounts are linked to a troll farm in St. Petersburg, referred to as the Internet Research Agency, though we have no way to independently confirm.”

Alex Stamos, Facebook’s chief security officer, said in a statement that the company was committed to protecting the integrity of its site and improving its ability to identify fraudulent accounts.