LONDON — Facebook said on Wednesday it was reopening and broadening an internal investigation into the possibility that Russia had used the platform to influence the British vote to leave the European Union.

Reports that the Kremlin had used Facebook and other social media to try to sway elections in the United States, France and other countries have raised widespread suspicions that it had played the same game in the June 2016 British referendum. Russia has long viewed the European Union as a threat, and promoting the groups that favored Britain’s leaving would be one way to weaken the bloc.

But the possibility of Russian interference has threatened to further complicate the torturous negotiations with Brussels over Britain’s departure, known as Brexit, by undermining the credibility of the referendum.

After the British Parliament asked Facebook, Twitter and other social media companies last year for information on any Russian efforts to sway the vote, Facebook said in December that it had found no evidence of manipulation.