WASHINGTON (Reuters) – Executives from Facebook, Alphabet Inc’s Google and Twitter have been asked to testify to the U.S. Congress in coming weeks as lawmakers probe Russia’s alleged interference in the 2016 U.S. election, committee sources said on Wednesday.

A Senate aide said executives from the three firms had been asked by the Senate Intelligence Committee to appear at a public hearing on Nov. 1.

The leaders of the House of Representatives Intelligence Committee said the panel would hold an open hearing next month with representatives from unnamed technology companies in an effort to “better understand how Russia used online tools and platforms to sow discord in and influence our election.”

Representatives for Facebook and Google confirmed they had received invitations from the Senate committee but did not say whether the companies would attend. Twitter did not immediately respond to requests for comment.

The House panel did not immediately identify any companies, but a committee source said lawmakers expected to hear from the same three firms the Senate had asked to testify.

The requests are the latest move by congressional investigators to gain information from internet companies as they probe the extent of Moscow’s alleged efforts to disrupt last year’s U.S. election. Lawmakers in both parties have grown increasingly concerned that social networks may have played a key role in Russia’s influence operation.

Facebook revealed this month that suspected Russian trolls purchased more than $100,000 worth of divisive ads on its platform during the 2016 election cycle, a revelation that has prompted calls from some Democrats for new disclosure rules for online political ads.

On Wednesday, Trump attacked Facebook in a tweet and suggested the world’s largest social network had colluded with other media outlets that opposed him. The president has been skeptical of the conclusions of U.S. intelligence agencies that Russia interfered in the election and has denied his campaign colluded with Moscow.

The salvo prompted a lengthy rebuke from Facebook Chief Executive Mark Zuckerberg, who said both Trump and liberals were upset about ideas and content on Facebook during the campaign.

“That’s what running a platform for all ideas looks like,” Zuckerberg wrote on his personal Facebook page.

Other internet firms besides Facebook are also facing rising scrutiny over how Russia may have leveraged their platforms. Twitter is expected to privately brief the Senate panel on Thursday.

Republican Senator James Lankford, who has received classified information about Russia’s interference as a member of the Senate Intelligence Committee, said on Wednesday that the country’s attempts to sow discord in U.S. domestic affairs had not abated.

Russian internet trolls over the weekend fueled the debate ignited by Trump over whether NFL players should have the right to kneel during the national anthem, Lankford said.

Also on Wednesday, the Daily Beast, citing unnamed sources, reported that a Facebook group named “United Muslims of America” was a fake account linked to the Russian government and that it was used to push false claims about U.S. politicians, including Democratic presidential candidate Hillary Clinton.

The group bought Facebook ads to reach targeted audiences, promoting political rallies aimed at Muslims, the website reported.

The Senate and House intelligence committees are two of the main congressional panels probing allegations that Russia sought to interfere in the U.S. election to boost Trump’s chances at winning the White House, and possible collusion between Trump associates and Russia.