(Rewrites throughout, adds Warner quotes, Twitter declining to comment)
By Dustin Volz and Jonathan Landay
WASHINGTON, Sept 7 (Reuters) - Twitter Inc is expected to brief U.S. congressional investigators soon on whether Russia used its advertising platform to promote divisive social and political messages during the 2016 election, Senator Mark Warner said on Thursday.
The news came a day after Facebook said an operation likely based in Russia had placed thousands of U.S. ads with polarizing views on topics such as immigration, race and gay rights on the social media site during a two-year-period through May 2017.
Warner, the top Democrat on the Senate Intelligence Committee, said the suspected Russian placement of such ads may have gone far beyond what Facebook disclosed, and that Twitter and other technology companies should also examine the issue.
"It was my belief that the Russians were using those sites to interfere in our elections, and the first reaction from Facebook was, ‘No. You're crazy.’" Warner said at the Intelligence and National Security Alliance conference in Washington.
"I think what we saw yesterday in terms of their brief was the tip of the iceberg," Warner said.
He also told reporters he expected Twitter to soon brief the Senate Intelligence Committee, one of the panels investigating Russian meddling in the 2016 election and whether members of President Donald Trump's campaign colluded with Moscow.
A Warner spokeswoman later confirmed that was expected to occur.
Twitter declined to comment.
Facebook briefed U.S. lawmakers on the issue on Wednesday and also turned over information about the ads to Robert Mueller, the special counsel who is leading his own investigation into alleged Russian interference in the election, a source familiar with the matter told Reuters.
That information included copies of the ads and data about the buyers, the source said.
Warner said he wanted to have Facebook brief lawmakers again and that he wanted Twitter and other companies to do the same.
"As you see for example in the case of Facebook, they denied that they were being used in any way. They didn’t do anything," Warner said. "But by the time of the French elections, Facebook was working with the French" and they shut down 50,000 accounts.
Warner said legislation may be required to change how social media platforms can be used for political advertising to bring federal disclosure rules in line with those governing television advertising. (Reporting by Dustin Volz and Jonathan Landay; Writing by Susan Heavey; Editing by Paul Simao)