Much of the trial has focused on Mr. Zarrab’s testimony, in which he implicated Mr. Erdogan in the sanctions-evasion scheme and described how he had paid tens of millions of dollars in bribes to Zafer Caglayan, then Turkey’s economy minister.
Mr. Zarrab, a flashy gold trader who is married to a Turkish pop star and who often appeared in Turkish celebrity magazines, said he alone had made as much as $150 million in the scheme.
Mr. Korkmaz, a slight man of 30 years old, was anything but flashy as he testified for the prosecution and admitted that he had known little about Mr. Zarrab when the investigation began.
“The rest of the team knew him through magazine news, through entertainment news,” he said. “I was not into those.”
Mr. Korkmaz, testifying through an interpreter, described the investigation’s findings, many of which are already known in Turkey. He cited the discovery of bribe payments delivered in shoe boxes and a discussion by Mr. Zarrab and Mr. Caglayan about delivering money to Mr. Erdogan’s son.
His most riveting testimony was about being a police officer who did not want an investigation to disappear.
Days after the arrests, he and others on his team were reassigned, he said, and told that they did not have the authority to investigate the government ministers or Mr. Erdogan. He said he had prepared reports of the investigation and had them delivered to a prosecutor and to Parliament.
After being released from prison in February 2016, he said, he did not feel “legally secure in any way,” and he fled Turkey that August. He took as much of the evidence as he could, obtained from a prosecutor and another investigator. “Both the prosecutor and I believed that the evidence would never be brought up in court” and would probably be “damaged or destroyed.”
“I took initiative in order to preserve the evidence,” he testified.
He described a complex journey, using a false passport, through several countries before he arrived in the United States, with the help of American law enforcement officials.
He told the jury that he believed that in Turkey, the right to defend one’s freedom had been lost. “So I took my wife and my daughter,” he said, “and I left the country that I dearly love.”