ISTANBUL (Reuters) - Turkish police have detained 17 people as part of an investigation related to gold trader Reza Zarrab, who is cooperating with prosecutors in a U.S. trial, state-run Anadolu news agency said on Tuesday.

Police detained three of Zarrab’s employees on suspicion of delivering documents from the network of Muslim cleric Fethullah Gulen to U.S. prosecutors, Anadolu said.

Ankara says U.S. prosecutors have based their case on documents fabricated by Gulen’s followers, who it also blames for orchestrating last year’s failed coup in Turkey.

Zarrab, a Turkish and Iranian national, has pleaded guilty to charges that he schemed to help Iran evade U.S. sanctions, and is testifying for U.S. prosecutors against an executive from a Turkish state-owned bank for related charges.

The executive has pleaded innocent and the bank has said all of its transactions complied with international regulations.

The case has strained ties between the United States and Turkey, both members of the NATO military alliance. It has infuriated President Tayyip Erdogan, who has cast the trial as an attempt to undermine Turkey and its economy.

“It is right that there is a plot allegation in the trial’s indictment, but it is a plot against Turkey, not against the United States,” Erdogan told members of his ruling AK Party in parliament.

“This trial is an international coup attempt, a process at the center of which is FETO,” he said, using the Turkish government’s term for Gulen’s network. Gulen, who has lived in the United States since 1999, has denied charges he was behind the coup, and condemned the military intervention.

Police searched the houses of the three employees and found electronic copies of documents, Anadolu said. It said fourteen more people had been detained as part of the investigation.

No one was immediately available for comment at Istanbul police headquarters.

Last week the Istanbul prosecutor’s office decided to seize the assets of Zarrab and those of his acquaintances as part of the investigation, Anadolu reported.