Through his lawyer, Michael Volkov, Colonel Vindman declined to comment.

Mr. Hickman, a former lieutenant colonel whose service record indicates he served in Afghanistan and earned a Purple Heart, at some point took an interest in QAnon. A review of his past tweets found more than 100 in which he recirculated or commented on QAnon-related theories, including hoaxes about Satanism and pedophilia, and until recently he had the hashtag #Q in his profile. Reached for comment, Mr. Hickman said he did not believe in QAnon but found it “interesting.”

“I do think it’s actually been pretty accurate on predicting a lot of things,” he said.

He has also tweeted strident pro-Trump, anti-Democratic themes, writing, “It’s incredible how evil the Democrat party is.” A week before going public with his story about Colonel Vindman, he retweeted a Trump supporter urging: “STOP IMPEACHMENT! STOP THIS COUP!”

In a Twitter thread, Mr. Hickman, who said he was disabled from combat injuries and living in Florida, said he had helped manage joint exercises in Germany involving United States and Russian soldiers. He met Colonel Vindman there in 2013, he said.

Colonel Vindman referred to himself as a patriot during closed-door testimony in the House last month, and said he had reported concerns about the president and his inner circle’s conduct out of a “sense of duty.” The colonel received a Purple Heart after being injured by an improvised explosive device in Iraq. He now serves on the National Security Council.

Several officials have publicly defended the colonel since his testimony emerged. General Joseph F. Dunford Jr., the former chairman of the Joint Chiefs of Staff, has called the colonel “a professional, competent, patriotic and loyal officer.” Michael McFaul, the former ambassador to Russia, has said he had worked with the colonel “and interacted with him in front of Russian officers. He never once said anything near what this ‘retired Army officer’ claims.”

Mr. Hickman appears to have first shared his story in a private “DM room” on Twitter, where people can send direct messages to one another. He said in a tweet that he had forgotten about the encounter with Colonel Vindman, but that his Army friends “reminded me of what happened and it all came back,” adding “Damn TBI,” a reference to traumatic brain injury.

As the tale gained attention on Twitter, and received pushback from some who questioned it, a new Twitter account popped up with the name Thomas Lasch, tweeting that he had worked with Mr. Hickman and remembered the 2013 episode.