K.T. McFarland, President Trump’s former deputy national security adviser, during her confirmation hearing to be ambassador to Singapore in July.

WASHINGTON — A leading Democrat on the Senate Foreign Relations Committee questioned on Monday whether a high-ranking official in Donald J. Trump’s transition team had been deceptive over the summer about her knowledge of discussions between Michael T. Flynn, the former national security adviser, and a former Russian ambassador.

K. T. McFarland served on the presidential transition team before becoming the White House deputy national security adviser. In July, she was questioned in writing by Senator Cory Booker, Democrat of New Jersey, on whether she had ever spoken to Mr. Flynn about his contacts with Sergey I. Kislyak, who was then the Russian ambassador to Washington, before Mr. Trump took office.

“I am not aware of any of the issues or events described above,” Ms. McFarland wrote in response, sidestepping a direct answer to the question.

An email exchange obtained by The New York Times indicates that Ms. McFarland was aware at the time of a crucial Dec. 29 phone call between Mr. Flynn and Mr. Kislyak that was intercepted by American intelligence. During that call, Mr. Flynn urged Moscow to respond cautiously to sanctions just imposed by the Obama administration for Russia’s interference in the presidential election.

Mr. Flynn pleaded guilty on Friday to lying to F.B.I. agents about that conversation and other interactions with Mr. Kislyak. He promised to cooperate with the special counsel, Robert S. Mueller III, who is investigating possible collusion between Russia and the Trump campaign and obstruction of justice by Mr. Trump.

If senators on the Foreign Relations Committee find that Ms. McFarland was evasive in her testimony, it could complicate her nomination to become ambassador…