The lawyers said their new client, whose identity is not known, is an intelligence official who had grown alarmed by the president’s behavior and was among those interviewed to corroborate the allegations of the original whistle-blower.

The Trump administration’s investigation: Mr. Trump’s attorney general, William Barr, is looking into the veracity of Russia’s involvement in the 2016 election. He traveled to Rome last month with a top federal prosecutor to look into one of Mr. Trump’s pet conspiracy theories: that some of America’s closest allies plotted with his “deep state” enemies to try to foil his election.

Despite criticism that he was doing the president’s bidding, the attorney general has signaled that the investigation is a priority and that he is personally overseeing it.

Background: Mr. Barr was in Italy to interview officials about Joseph Mifsud, a professor whose actions figured prominently in the F.B.I.’s rationale for opening the Russia inquiry. Here’s how Mr. Mifsud fits into the story.