An early portrait of the intelligence officer began to take shape on Wednesday as the White House released a rough log of a July 25 phone call between Mr. Trump and President Volodymyr Zelensky of Ukraine, the latest extraordinary revelation set off by the whistle-blower’s complaint.

This account is based on interviews with the two people and with lawmakers who were permitted to read the complaint late in the day, as well as on details revealed in a Justice Department memo explaining the Trump administration’s legal rationale for withholding the whistle-blower’s allegations from Congress before Mr. Trump relented this week. The White House did not respond to a request for comment.

Mr. Atkinson also found reason to believe that the whistle-blower might not support the re-election of Mr. Trump and made clear that the complainant was not in a position to directly listen to the call or see the memo that reconstructed it before it was made public, according to the Justice Department memo, which referred only to a single phone call between Mr. Trump and an unnamed foreign leader .

Instead, the officer heard about the call secondhand from unidentified White House officials who expressed concern that Mr. Trump had “abused his authority or acted unlawfully in connection with foreign diplomacy,” the memo said. Still, Mr. Atkinson concluded after an investigation that the information in the complaint was credible.

In their first public comments, lawyers for the whistle-blower said their client hoped to remain anonymous but wanted to continue to cooperate with lawmakers conducting oversight.