Senator Ron Wyden of Oregon, the ranking Democrat on the Finance Committee, said Friday that members of the delegation “may have violated U.S. sanctions,” a notion Republicans refuted.

While the N.R.A.’s leadership has tried to distance itself from the 2015 trip, email traffic and a flurry of transactions reviewed by The New York Times earlier this week undercut claims that Mr. LaPierre and other leaders were not supportive. In the months after the trip, facing unpaid expenses of more than $21,000, N.R.A. officials had one prominent member of the delegation, Pete Brownell, write checks to pay costs related to the trip, but also transferred the same amount of money back to Mr. Brownell. The circuitous transactions later raised internal legal concerns among N.R.A. lawyers.

The series of transactions were overseen by a close aide to Mr. LaPierre, who said in a 2016 internal email obtained by The Times that they were approved by Mr. LaPierre. The N.R.A. said this week that the aide had misrepresented Mr. LaPierre’s support.

The Democrats’ new report cast further doubt on the N.R.A.’s efforts to disavow the trip, highlighting a letter from Allan Cors, then the N.R.A.’s president, on N.R.A. letterhead, in which he wrote that two of the trip’s attendees “will represent the N.R.A. and its five million members.” The letter was written to Alexander Torshin, a political patron of Ms. Butina.

Andrew Arulanandam, a spokesman for the N.R.A., said, “Certain N.R.A. members made the trip of their own accord. The record reflects it was not an official NRA trip.”

Democrats on the committee said they would pursue a second investigation examining the N.R.A.’s broader spending practices, and also planned to refer some of the issues raised in the report to the Internal Revenue Service, which is seen as unlikely to take action.

“During the 2016 election, Russian nationals effectively used the promise of lucrative personal business opportunities to capture the N.R.A. and gain access to the American political system,” Mr. Wyden said in a statement. “The totality of evidence uncovered during my investigation, as well as the mounting evidence of rampant self-dealing, indicate the N.R.A. may have violated tax laws.”