Ms. Hill grew alarmed earlier this year when an aide from the White House executive secretarys office told her that Mr. Trump wanted to talk to Mr. Patel and identified him as the National Security Councils Ukraine director, a position held by one of Ms. Hills deputies. The aide said Mr. Trump wanted to meet with Mr. Patel about documents he had received on Ukraine.

Ms. Hill responded by asking who Mr. Patel was. While the aide from the executive secretarys office did not state explicitly that Mr. Patel sent the Ukraine documents to Mr. Trump, Ms. Hill understood that to be the implication, according to a person familiar with her testimony.

Mr. Patels apparent communications with the president prompted Ms. Hill to raise concerns with her superiors, including John R. Bolton, then the national security adviser, that Mr. Patel was meddling outside of his portfolio. As early as May, Ms. Hill had begun discussing with colleagues her concerns about whether Mr. Patel was running a shadow effort on Ukraine at the White House, according to four people briefed on the discussions.

Two other witnesses in the impeachment investigation, Mr. Sondland and George P. Kent, another State Department official, testified that they had not encountered him in their work, according to people familiar with the inquiry.

Mr. Patel joined the National Security Council in February and began getting involved in Ukraine matters in April, as Mr. Giuliani pushed the Ukrainian government to discredit evidence against Paul Manafort, Mr. Trumps former campaign chairman. It was not clear who hired him.

Mr. Patel was previously best known as a lead author of the politically charged memo released early last year accusing the F.B.I. and Justice Department leaders of abusing their power in the early stages of the Russia investigation.

Mr. Patel worked at the time as an investigator for the House Intelligence Committee under Representative Devin Nunes of California, who ran the panel when Republicans had control of the chamber. Mr. Patels efforts to discredit the Russia investigation made him a minor celebrity in conservative circles but a divisive figure on Capitol Hill.