Mr. Mashburn’s testimony was described by three people familiar with it, who spoke on the condition of anonymity because they were not authorized to discuss the session publicly. The Judiciary Committee, like other panels on Capitol Hill, is conducting its inquiry mostly in secret and has publicized little of what it has learned.

Lawyers for Mr. Mashburn and Mr. Papadopoulos declined to comment. Michael Glassner, the chief operating officer of the campaign, and Mr. Mashburn did not reply to requests for comment.

A veteran culture warrior and anti-abortion activist, Mr. Mashburn joined the Trump campaign in April 2016 as Mr. Trump was wrapping up the Republican nomination and looking to consolidate support after a bruising primary. A longtime Senate lawyer — he rose to prominence as an aide in the 1980s to Senator Jesse Helms of North Carolina, the divisive conservative firebrand — Mr. Mashburn brought with him deep connections to Washington’s conservative policy networks and a long résumé of the sort that had been lacking on the campaign.

For about the next six months, Mr. Mashburn served as the campaign’s policy director, working closely with Rick Dearborn, another longtime Senate lawyer, to oversee the campaign’s policy shop in Washington. Later, he advised the presidential transition team and followed Mr. Dearborn to the White House, where until mid-April he was the deputy cabinet secretary at the White House. Mr. Mashburn is now a senior adviser to Rick Perry, the energy secretary.

During his testimony before the Judiciary Committee, Mr. Mashburn said repeatedly that he recalled receiving a message with some detail about Russian information on Mrs. Clinton, and that other campaign officials almost certainly would have been copied on the memo, the people said. Mr. Mashburn did not name the campaign officials and did not remember precisely when he received the message.

Mr. Mashburn told the investigators that he did not take Mr. Papadopoulos, a volunteer adviser with few political connections and scant experience, seriously. He said he did not remember acting on the communication, according to the people familiar with his testimony. At one point in the interview, he recalled replying to Mr. Papadopoulos to instruct him to raise the issue with someone else, though he did direct him to a particular campaign aide. At another point, he said he might have forwarded the email to Mr. Dearborn.