[Update: Cesar Sayoc was sentenced to 20 years in prison.]

Speaking in a halting, raspy voice, Cesar A. Sayoc Jr., sat in a Manhattan federal courtroom on Thursday and described how he painstakingly assembled homemade pipe bombs that he sent to prominent Democrats, including Hillary Clinton, former President Barack Obama and other critics of President Trump last fall.

Each device, Mr. Sayoc said, consisted of a plastic pipe with a digital alarm clock and attached wires. He packed all of them with a potentially deadly mix: powder from fireworks, fertilizer, a pool chemical and glass fragments. Finally, he inserted into each package a photograph of the intended recipient, marked with a red X over the face.

Mr. Sayoc, 57, paused his explanation and broke into sobs, finally collecting himself and speaking softly just before he pleaded guilty to the attack. “I know these actions were wrong, and I’m extremely sorry,” he said.

It was his first public explanation of the bombing campaign that gripped the nation just before the midterm elections in October. Mr. Sayoc mailed 16 devices to his intended victims around the country, who also included the actor Robert De Niro and CNN. After a four-day manhunt, he was arrested in Florida, where he appeared to be living in a white van plastered with conservative slogans and images.