A trial over a conspiracy to violate United States sanctions with a billion-dollar scheme to smuggle Turkish gold for Iranian oil began in New York this week.

The case has riveted Turkey and rattled the Turkish government.

Before the trial opened, there was broad speculation that the lead defendant, Reza Zarrab, had entered a plea bargain with the prosecution and would reveal damaging information about corruption and illegal dealings in high places.

The other defendant, Mehmet Hakan Atilla, a Turkish banker, pleaded not guilty before the trial began. Seven others have been charged in the conspiracy but remain at large.

Here’s what we’ve learned in the trial’s first week.

A surprise witness was revealed, and a mystery solved.

A month ago, Mr. Zarrab, 34, a Turkish-Iranian gold trader, was facing years in prison on charges that he had helped orchestrate the billion-dollar scheme in violation of American sanctions.