German authorities are investigating 20 Turkish citizens on suspicion of conducting espionage in Germany, a newspaper reported on Thursday.

Die Welt said it received the information in an official government response to a question about the issue filed by Sevim Dagdelen, a German lawmaker with the hard left party Die Linke.

Tensions are running high between the two NATO allies ahead of a referendum in Turkey next month that proposes expanding the powers of President Tayyip Erdogan. Berlin infuriated Ankara after cancelling several campaign rallies by Turkish ministers in Germany, drawing accusations from Turkey of “Nazi” tactics.

Germany’s Federal Prosecutor’s Office (GBA) said last month it had launched a probe into suspected spying by Turkey. German media said at the time that the entity being investigated was the Turkish Intelligence Agency (MIT) and that it was suspected of spying on supporters of U.S.-based cleric Fethullah Gulen. Ankara accuses Gulen of organizing a failed coup last July, a charge he denies.

“At the moment we are investigating a total of 20 suspects on suspicion of conducting secret service agent activities on behalf of the Turkish government,” the German government wrote in its answer to the German lawmaker, Die Welt said.

German police in February raided the apartments of four imams suspected of spying on followers of Gulen on behalf of the Turkish government.

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