The Commission on Presidential Debates said on Friday that Donald Trump's microphone led to problems with the audio transmitted into the auditorium at Hofstra University on Monday evening.
"There were issues regarding Donald Trump's audio that affected the sound level in the debate hall," the commission said in a one-sentence statement posted to its website.
It made no mention of the audio for the television broadcast, which was seen by more than 83 million people and gave no other details.
In the spin room Monday night, the Republican presidential candidate cited problems with his microphone.
"Did you notice that? My mic was defective within the room," he said. "I wonder, was that on purpose? Was that on purpose? But I had a mic that wasn't working properly within the room."
Since then, Trump has claimed that the debate was "rigged," pointing to both the mic and the moderator, Lester Holt of NBC News.
"I had to put up with the anchor and fight the anchor all the time on everything I said. What a rigged deal," he said at a campaign rally in New Hampshire Thursday afternoon.
On Tuesday, Hillary Clinton ridiculed his complaints as a sign that he knew he fell short on the debate stage.
"Anybody who complains about the microphone is not having a good night," she told reporters in response.
He also faced questions for repeatedly making a sniffling noise during the debate, leading some viewers to wonder if he had a cold. "No, no sniffles, no cold," he said.
Trump has pitched his stamina as an important qualification for the White House, drawing a contrast with Clinton, whose health scare left her unable to stand up on her own at a public event on Sept. 11. Her aides later said she'd been diagnosed with pneumonia. She took three days off the campaign trail to recover.
On Wednesday, Trump's deputy campaign manager David Bossie said the sound was due to a turned-up mic picking up Trump's normal breathing. "That's really what it was," he said on Fox.