Huseyin Korkmaz, a Turkish police supervisor in Istanbul, began an investigation in 2012 into a money laundering and bribery ring led by Reza Zarrab, a Turkish-Iranian businessman who was secretly smuggling gold for oil in violation of United States sanctions on Iran.

The investigation broadened to include several government ministers, the general manger of a Turkish state bank and Recep Tayyip Erdogan, then Turkey’s prime minister and now its president, Mr. Korkmaz testified in a Manhattan courtroom on Monday.

In a dawn raid on Dec. 17, 2013, Mr. Zarrab, the banker and the businessmen sons of three ministers were arrested on corruption and bribery charges. But the Turkish government stepped in quickly to release them, and quashed the investigation. Three dozen police officers and officials were purged within days. Mr. Korkmaz was reassigned, sent to guard a bridge, he testified, and was later jailed.

Mr. Korkmaz was eventually released and later had himself smuggled out of Turkey. He made it to the United States with his wife and daughter, carrying a trove of evidence from the corruption investigation.