Facebook denied a request from Joe Biden's campaign to take down a video ad by President Trump's reelection campaign that falsely accuses Biden of corruption in Ukraine policy under the Obama administration.

In the letter obtained by CNN, Facebook said the decision is "grounded in Facebook's fundamental belief in free expression, respect for the democratic process, and belief that in mature democracies with a free press, political speech is already arguably the most scrutinized speech there is."

Facebook added that they do not send ads to third party fact-checkers. The New York Times first reported the letter from Facebook.

The Trump campaign ad: The video accuses Biden of using his power as then-vice president to influence Ukraine in order to benefit his son, Hunter. There is no evidence of wrongdoing by Joe or Hunter Biden. CNN has refused to air the ad.

The Biden campaign responded to the Facebook letter by calling the decision "unacceptable."

Some context: Trump has repeatedly claimed that Biden had called for the firing of a Ukrainian prosecutor who was "investigating his son."

There is no evidence Hunter Biden was ever under investigation. The investigation was into the business dealings of the owner of a Ukrainian natural gas company, Burisma Holdings, where Hunter Biden sat on the board of directors.

Read more about it here.