WASHINGTON — The whistle-blower who revealed that President Trump sought foreign help for his re-election and that the White House sought to cover it up is a C.I.A. officer who was detailed to work at the White House at one point, according to three people familiar with his identity.

The man has since returned to the C.I.A., the people said. Little else is known about him. His complaint made public Thursday suggested he was an analyst by training and made clear he was steeped in details of American foreign policy toward Europ e, demonstrating a sophisticated understanding of Ukrainian politics and at least some knowledge of the law.

The whistle-blower’s expertise will likely add to lawmakers’ confidence about the merits of his complaint, and tamp down allegations that he might have misunderstood what he learned about Mr. Trump. He did not listen directly to a July call between Mr. Trump and President Volodymyr Zelensky of Ukraine that is at the center of the political firestorm over the president’s mixing of diplomacy with personal political gain.