The identity of the other man was unclear. Mr. Bucher works for the Security Administration of the F.C.C., according to public records. A request to the commission for an interview with him went unanswered.

Mr. Donnelly said the article he was developing was unrelated to the hearing, and he wanted to ask his question out of earshot of other reporters at the news conference. It is common practice in Washington and other government settings for reporters to ask questions or attempt one-on-one interviews outside a press gaggle to protect an exclusive story.

He said the officials were “up in my face” and made clear verbally and in their body language that they wanted him out of the building. When Mr. Donnelly left and sat in a wooden chair in the lobby, Mr. Bucher approached. After some back and forth about why Mr. Donnelly was still there, the reporter said he was told he would have to leave.

In an exchange on Twitter, Mr. O’Rielly, the commissioner, wrote that he did not recognize Mr. Donnelly. “John, I saw security put themselves between you, me and my staff,” he wrote. He said he “didn’t see anyone put a hand” on Mr. Donnelly but that he didn’t doubt his account. Mr. O’Rielly apologized and added, without elaboration, that he was also “freezing and starving.”

“I appreciate the apology,” Mr. Donnelly replied. “But ‘put themselves’ there makes it sound dainty. They pinned me.”

Mr. O’Rielly, who was appointed to the commission by President Obama in 2013, did not respond to an email and a tweet seeking comment. The F.C.C. did not respond to questions about what, if any, changes might be made by security officials after the episode.

Two Democratic senators, Tom Udall of New Mexico and Maggie Hassan of New Hampshire, sent a letter on Friday to the commission demanding a full explanation of what happened and seeking assurances that security staff would not restrain or threaten journalists in the future. They called it “a new low point in a disturbing trend” under the Trump administration.