In a statement on Friday night, Fox Business emphasized that the clips it used of Ms. Pelosi did not slow down her speech.

On Friday, Mr. Giuliani said that he did not know that the distorted video that he shared had been altered when he shared it on Twitter. “I didn’t know it was doctored,” he said. “I had no reason to believe it at the time. It looked like enough of an extension of the way she communicates anyway.”

“It did seem a little exaggerated and I think I tweeted, ‘What’s wrong?’” he added. “But to overreact is a little hypocritical given she is the one who was making very, very direct comments about the competence of the president of the United States of America, which I don’t think any good American should do.”

Mr. Giuliani said that he took the tweet down after someone texted him calling the video into question. He said he had not seen the original footage.

“Where do you go to check that it’s inaccurate?” he said. “How could I have figured out that it was inaccurate?”

Dr. Lyu, who has studied deepfakes, a kind of ultrarealistic fake video made with artificial intelligence software, said that many false videos can be detected if people slow down, watch again and think critically.

“There is no way back; the Pandora’s box is opened,” he said. But he added: “We are all part of the ecosystem, consuming and also generating information, so we must do our part of the job to make the ecosystem healthy.”