WASHINGTON (Reuters) - Special counsel Robert Mueller's team has questioned Sam Clovis, co-chairman of President Donald Trump's election campaign, to determine if Trump or top aides knew of the extent of the campaign team's contacts with Russia, two sources familiar with the investigation said on Friday.

The focus of the questions put to Clovis by Mueller's team has not been previously reported.

"The ultimate question Mueller is after is whether candidate Trump and then President-elect Trump knew of the discussions going on with Russia, and who approved or even directed them," said one source. "That is still just a question."

Clovis testified in late October before the grand jury in Mueller's investigation into Russian interference in the 2016 presidential election. He is also cooperating with the Senate Intelligence Committee, which is investigating the same issues.

Contacted late on Friday, the White House declined to comment.

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One of the sources described Clovis as "another domino" after former campaign foreign policy adviser George Papadopoulos pleaded guilty to lying to the FBI over his own contacts with Russians during the 2016 election campaign.

"The investigators now know what Papadopoulos was doing on the Russian front, which he initially tried to conceal, and who he told that to," said the other source. "Now [they] want to know whether Clovis and others reported these activities and others related to Russia, and if so, to whom," this source said.

Attorneys for Clovis did not respond to requests for comment. Lawyers for Papadopoulos had no immediate comment.

Peter Carr, a spokesman for Mueller, declined to comment.

According to court documents related to Papadopoulos' guilty plea, he reported to Clovis in an email on a March 24, 2016, meeting he had in London with a professor later identified as Joseph Mifsud.

Mifsud in turn introduced him to a Russian woman and the Russian ambassador in London, and they discussed setting up meetings to talk about U.S.-Russia ties in a Trump presidency.

The documents showed Clovis responded to the proposed meetings by saying he would "work it through the campaign." While he told Papadopoulos not to make a commitment then to set up those meetings, he congratulated him for "great work."

In August 2016, after Trump won the Republican presidential nomination, Clovis encouraged Papadopoulos to "make the trip" when Papadopoulos proposed going to an off-the-record meeting with unnamed Russian officials, the court documents show.

Victoria Toensing, one of Clovis's lawyers, said last week her client "always vigorously opposed any Russian trip for Donald Trump and/or the campaign".

After Papadopoulos' guilty plea, the White House and former Trump campaign officials dismissed Papadopoulos and Clovis as minor figures in the campaign.

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The campaign's National Security Advisory Committee, which Clovis formed, has become a focus of the investigations by both Mueller and the Senate, sources said.

"Sam built the first group of eight," J.D. Gordon, the director of the campaign foreign policy group, told Reuters, adding that he and then-Senator Jeff Sessions, now the U.S. Attorney General, had "nearly doubled" it in size.

However, two other sources familiar with the investigations said investigators have been told the committee Clovis formed did very little, and that other advisers appeared to carry more weight with Trump.

(Reporting by Mark Hosenball, Jonathan Landay and John Walcott; Writing by John Walcott; Editing by Kieran Murray and Clarence Fernandez)