ISTANBUL - Turkish authorities have opened an investigation into the US prosecutors who brought charges against a Turkish gold trader facing trial in New York, state media said on Saturday, after Ankara said the case was based on fabricated documents.The Istanbul prosecutor's office is investigating former US Attorney Preet Bharara and acting U.S. Attorney John H. Kim, the state-run Anadolu news agency said, following allegations that their case was based on documents Turkey says were fabricated.When asked for comment, Bharara referred Reuters to the Southern District of New York's Attorney's Office. James Margolin, a spokesman for the Attorney's Office, declined to comment.Reuters was not immediately able to reach anyone at the Istanbul Prosecutor's office for comment.The reported move comes after Turkish Foreign Minister Mevlut Cavusoglu on Friday said the U.S. case was based on documents fabricated by followers of the cleric Fethullah Gulen, whom Ankara blames for last year's attempted coup. Cavusoglu also accused Bharara of being "very close" to Gulen's network.Following those allegations, Bharara responded on Friday on Twitter, saying: "Turkey FM is a liar. Now let's see what happens in court."The case against the wealthy, Iran-born gold trader Reza Zarrab has complicated already strained relations between the United States and Turkey, both members of the NATO military alliance. Zarrab, together with alleged co-conspirators, has been charged with handling hundreds of millions of dollars for Iran's government and Iranian entities from 2010 to 2015, in a scheme to avoid sanctions.He has pleaded not guilty and is due to go on trial in New York on Nov. 27.Under a previous Turkish investigation that became public in 2013, Turkish prosecutors accused Zarrab and high-ranking Turkish officials of involvement in facilitating Iranian money transfers via gold smuggling, leaked documents at the time showed.President Tayyip Erdogan, then prime minister, cast that investigation as a coup attempt orchestrated by his political enemies. Several prosecutors were removed from the case, police investigators were reassigned, and the investigation was later dropped.Erdogan, who has not been accused of any wrongdoing, has said US prosecutors have "ulterior motives" by including references to him and his wife in court papers relating to the trial in New York.