Reza Zarrab, a Turkish-Iranian gold trader at the center of an international corruption case, has agreed to cooperate with the American prosecution of a Turkish banker accused of helping Iran evade U.S. sanctions, a defense lawyer said at the start of the trial.

Zarrab is the U.S. government’s “most important witness,” defense attorney Robert Fettweis told a judge as opening statements were set to begin in Manhattan federal court. His client, Mehmet Hakan Atilla, a deputy chief executive officer at Turkiye Halk Bankasi AS, is accused with conspiring with Zarrab and others.

Zarrab is likely to take the witness stand on Wednesday, U.S. District Judge Richard Berman said in court.

The disclosure of Zarrab’s cooperation resolves weeks of mystery after the gold trader suddenly stopped participating in the defense. With the U.S. expected to present evidence of corruption at the highest reaches of the Turkish government, the case has riled U.S.-Turkey relations and sparked outrage from Turkish president Recep Tayyip Erdogan, who claims the prosecution is a plot to undermine his country’s economy.

On Tuesday, Turkey’s lira fell the most among emerging markets -- as much as 0.9 percent -- on concern demand for the nation’s assets could be hurt by any fallout from the trial.

Zarrab was arrested in the U.S. last year and accused of working with bank executives and others in Turkey to help Iran disguise hundreds of millions of dollars in transactions funneled through the U.S. financial system. Zarrab, who is alleged to bribed Turkish officials to facilitate the scheme, may now testify about details of the scheme, as well as government corruption.

Mystery Deepens on Eve of U.S. Sanctions Case Roiling Turkey

Atilla is accused of fraud, sanctions violations and conspiracy to launder money. He’s one of nine people to have been charged, including senior Turkish government officials and banking executives, but only Zarrab and Atilla have been in U.S. custody. Prosecutors say they helped Iran transact business in dollars and access the global financial system at a time when the U.S. was ratcheting up sanctions on the country as punishment for its effort to build a nuclear bomb.

The revelation of Zarrab’s cooperation came as defense lawyers were seeking a brief postponement of the trial, complaining that prosecutors had just turned over 10,000 documents, many in Turkish, that they had not had time to review. Berman denied to request.

It is also possible Zarrab has pleaded guilty in the case. Berman on Tuesday agreed to unseal documents related to Zarrab’s “plea.” Such records may be publicly available later on Tuesday.

Since his arrest in Florida in March 2016, Zarrab had been held in grim New York City jails before he was suddenly released. Two people familiar with the case said Zarrab has remained in U.S. custody but not in the federal jails.