Senate intelligence committee investigators are interviewing former members of President Donald Trump's campaign as they hunt for evidence of possible collusion with Russia, asking one witness Friday fresh questions about the president's business dealings and how he formulated his policies toward Moscow.

Sam Nunberg, who worked for Trump and his campaign in 2015, said he was questioned in a closed-door session on Capitol Hill about Trump's trip to Moscow in 2013, his company's interest in building a tower there and specific relationships between past members of the campaign and foreign actors who may have worked with Russia.

"They are doing an exhaustive investigation," Nunberg told NBC News after his interview, which he said appeared to be "narrowly focused on collusion."

Nunberg's recollection of the day's events provided a firsthand account of the workings of committee investigators as Trump's presidency enters its third year.

Trump has strongly denied there was any collusion between his campaign and Russia. Special Counsel Robert Mueller is separately investigating the possibility of collusion.

Staff on the Senate committee, which is chaired by Republican Sen. Richard Burr of North Carolina, continued Friday in a closed door interview to press that line of inquiry.

Investigators went through a "check list" of questions, Nunberg said, including whether he had been aware of any conversations or relationships during his time with Trump regarding Russian banks, Russian oligarchs or business dealings with Russia.

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Nunberg, who sat with committee staff for four and half hours, said he was asked repeatedly about how Trump formulated his policy positions regarding Russia. Trump has voiced support for numerous foreign policy positions beneficial to Russian President Vladimir Putin. Nunberg said he told the committee that Trump, as a candidate, said "he would take the position that he was happy Russia was in Syria."

At the time, Nunberg said that position raised no red flags because he saw it as consistent with Trump's generally held view that the United States should not be involved in the Middle East. Nunberg said the campaign was getting questions at that time about how he saw U.S. involvement in Syria.

Also of interest to investigators, Nunberg said, was the campaign's relationship with the National Rifle Association and efforts by a Russian national to get a meeting with Trump through the NRA. Nunberg says he told investigators Friday that he was aware of efforts by Maria Butina, who pleaded guilty last month to conspiring with a Russian official to interfere in American politics, to seek a meeting with Trump, using the NRA as a conduit.

Investigators also peppered Nunberg with questions which suggested to him that they were trying to pin down specific relationships among members of Trump's campaign and organization and outside actors, including Wikileaks founder Julian Assange.

Last year, Nunberg went on a whirlwind media tour declaring that he wouldn't comply with a subpoena from the special counsel's office. But he ultimately reversed course and sat for interviews with Mueller's investigators. He also testified before a federal grand jury.

In the interview with the Senate Intelligence Committee, Nunberg said he was asked about numerous former campaign staff members, the president's children and other associates including: Roger Stone, Jerome Corsi, Paul Manafort, Michael Flynn, Tom Barrack, Michael Cohen, Steve Bannon, Donald Trump Jr and Eric Trump. He was also asked about Trump's relationship with Aras Agalarov, a Russian oligarch, and his pop-singer son, Emin, who helped set up the 2016 Trump Tower meeting.

Nunberg said he was not asked about the president's son-in-law and current White House adviser, Jared Kushner.

Sam Nunberg, former campaign aide for Donald Trump, exits federal court in Washington on March 9, 2018. Andrew Harrer / Bloomberg via Getty Images file

Among the other topics raised Friday, Nunberg said, were the 2016 meeting in Trump Tower in New York with Trump Jr, Manafort, Kushner and a Kremlin-connected Russian lawyer offering them the promise of "dirt" on Democratic candidate Hillary Clinton; Trump's travel to Moscow in 2013 for the Miss Universe pageant; the company's interest in building a tower in the Russian capital.

Nunberg said he was surprised by the interest in a Moscow tower since the campaign's official position was that it would not seek new foreign deals. Nunberg said he was asked to review public statements, emails, tweets and text messages obtained by investigators related to potential Russian interference.

Nunberg described the committee's investigation as professional and bipartisan. "If I were the White House, I would be concerned," said Nunberg, who joined the campaign early but was fired in August 2015 after racially charged Facebook posts were uncovered. He later apologized.

Unlike the House Intelligence Committee's Russia investigation, which was shuttered by Republicans last spring over the protest of Democrats, the Senate committee's probe has proceeded deliberately and in a largely non-partisan manner. Led by Burr and Ranking Democrat Sen. Mark Warner of Virginia, the panel has broken its probe into five parts and began releasing preliminary findings and recommendations last fall.

Those separate reports include: an analysis of the Intelligence Community's assessment of Russia's interference in the 2016 election; Russia's use of social media and how social media firms addressed the abuse of their platforms; how the FBI and Obama administration addressed Russia's activities and informed the public; an assessment of federal and state election security readiness; and whether Trump campaign officials colluded with the Russians.

The final question is potentially the most difficult for lawmakers on the committee to reach consensus on, and may not be fully answered until after the special counsel issues its final report.

Burr said Thursday he still hopes to hear additional testimony from Cohen, the president's former lawyer and fixer, who is set to appear publicly before the House Oversight Committee next month.

Cohen pleaded guilty last month to lying to Congress about interest in building a Trump tower in Moscow.

Flynn, who served as Trump's first national security adviser, separately pleaded guilty to lying to the FBI about contacts with the Russian ambassador and is cooperating with the special counsel. Manafort, who served as Trump's campaign chairman in 2016, pleaded guilty last year to one count of conspiracy and one count of obstruction of justice related to witness tampering.