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 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.
But lawmakers complained that Facebook’s internal research had been inadequate, and on Wednesday, the social media giant acknowledged that its previous investigation had focused only on the Russian-linked accounts already identified by United States intelligence agencies as having been active in the 2016 presidential election.
“Our investigatory team is now looking to see if we can identify other similar clusters engaged in coordinated activity around the Brexit referendum that was not identified previously,” Simon Milner, Facebook’s policy director for Britain, wrote in a letter to Parliament.
Facebook faces a greater challenge in identifying potential Russian activity around the Brexit vote — if any exists — because British intelligence agencies have not identified a list of suspected accounts, as the American agencies did.
“We would be very interested in receiving any information, including intelligence assessments or reports, that you and the U.K. government may have that is relevant to this work,” Mr. Milner said in his letter, noting that the inquiry “requires detailed analysis of historic data by our security experts.”
Academic researchers have analyzed information from other social media platforms such as Twitter, where data is more accessible, and a few have found indications of Russian activity but not conclusive evidence. Many have urged more disclosure by Facebook, Twitter and other companies, which possess internal information about users and payments.
A study by the Oxford Internet Institute concluded there was scarce evidence that the Kremlin had exploited the social media platforms to influence the British vote.
Damian Collins, the Conservative member of Parliament leading the inquiry, said in a statement, however, that “companies like Facebook should initiate their own research into issues like this, where there is such clear public concern, and not just act on intelligence that has been passed to them.”
“They are best placed to investigate activity on their platform.”