Desmond Mills Jr. watched the footage from his body camera, the Memphis courtroom silent except for the sounds of punches and Tyre Nichols’s agonized cries for his mother.

Then, Mr. Mills began to cry.

“I wish I could have stopped those punches,” Mr. Mills said through sobs during his testimony on Tuesday. “It hurt to watch. It hurt inside so much. I felt bad every time his picture is on the screen — to know I’m part of that.”

“I made his child fatherless,” he added. “I’m sorry.”

The apology from Mr. Mills, a former Memphis police officer who has pleaded guilty to federal charges, was the first explicit statement of remorse from any of the five officers charged in connection with Mr. Nichols’s beating and death in January 2023. Mr. Nichols, a 29-year-old FedEx worker, died three days after the beating, leaving behind a young son.

The dramatic admission came as prosecutors appeared to be nearing the end of their case against three of Mr. Mills’s former colleagues, who are on trial on charges of violating Mr. Nichols’s civil rights and conspiring to lie about it.