“That is an area of continuing interest for us,” Rep. Adam Schiff (D., Calif.), who is expected to become chairman of the House Intelligence Committee when Democrats regain control of the House in January, said in an interview. “In fact, that’s one of the areas that we’ve been able to make progress” on, despite Republican attempts to end the probe.

Mr. Schiff declined to detail what more the panel has learned. The Wall Street Journal has reported that the GOP activist, Peter W. Smith, who died in 2017, secretly raised $100,000 to secure emails deleted from Mrs. Clinton’s private computer server, and that he struck up a professional relationship with Mike Flynn, then a Trump aide, who later pleaded guilty in connection with special counsel Robert Mueller’s probe of Russian interference in the 2016 presidential election.

Another priority Mr. Schiff identified is to determine any financial relationship between the Trump Organization and Russian entities or individuals. Mr. Trump, his company and the Kremlin all have denied collusion or other improper ties.