The Commission on Presidential Debates said Friday that the first debate on Monday was marred by an unspecified technical malfunction that affected the volume of Donald J. Trump’s voice in the debate hall.

Mr. Trump complained after the debate that the event’s organizers had given him a “defective mike,” contributing to his widely panned performance against Hillary Clinton. Mrs. Clinton lampooned Mr. Trump’s claim, telling reporters on her campaign plane, “Anybody who complains about the microphone is not having a good night.”

Mr. Trump was clearly audible to the television audience. And there is no evidence of sabotage. But it turns out he was on to something.

“Regarding the first debate, there were issues regarding Donald Trump’s audio that affected the sound level in the debate hall,” the commission said in its statement.