Emmitt Martin III was already angry on Jan. 7, 2023, his nerves simmering in his first days back on patrol in Memphis after being sidelined for months by a car accident. As a member of a street crime unit, he said, he felt the need to prove he was still a capable officer, and he had yet to make an arrest.

Then he saw a blue Nissan speed up to beat a red light.

On Tuesday, Mr. Martin told a jury that he had not only pursued that driver, Tyre Nichols, a 29-year-old Black man, but that he had also punched and kicked him, and then lied about what happened. He identified the other officers there that night: all men with whom he had an unspoken understanding to never disclose the use of unwarranted force.

“I knew they weren’t going to tell on me,” Mr. Martin said. “And I wasn’t going to tell on them.”

Mr. Martin has since pleaded guilty to two charges in connection with Mr. Nichols’s beating and death three days later. On trial are three of his former Memphis police colleagues — Demetrius Haley, Tadarrius Bean and Justin Smith — who are accused of depriving Mr. Nichols of his civil rights and lying about what happened.

Mr. Martin’s testimony has pierced the officers’ code of silence and stripped the case to its core: whether a jury will believe that the violence and lies amounted to a federal crime.