"Turkey has tried on different levels to convince us of that fact, but they have not succeeded," foreign intelligence service chief Bruno Kahl told the German daily Der Spiegel.
Turkish authorities have blamed the attempted putsch that left 248 people dead on a rogue military group led by Gulen, a former Erdogan ally who has lived in self-imposed exile in the United States since 1999.
Gulen has strongly denied the accusations, but Ankara has repeatedly demanded his extradition.
"The putsch was just a welcome pretext," he added, referring to the unprecedented wave of purges Turkey has initiated since last summer.
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Turkish authorities have arrested more than 41,000 people, and fired or suspended 100,000, since the attempted coup. Many are teachers, police, magistrates or journalists.
Last month, authorities ordered the dismissal of nearly 4,500 civil servants, including 2,585 employees of the education ministry, 893 members of the gendarmerie and 88 workers at the public television channel TRT.
"The putsch was not initiated by the state," Kahl said. "Before July 15, a big purge by the government was already underway.
"That's why a part of the military thought they needed to quickly commit a coup before they were also affected (by the purges)," he said. "But it was too late and they too were also subject to purges."
Kahl said Gulen's movement is "a civil association of continuing religious and secular education" which "for decades has cooperated" with Erdogan -- and not a terrorist movement, as Ankara claims.
The movement promotes moderate Islam and runs a network of private schools in Turkey and the US, as well as a range of businesses, media outlets and cultural centres.
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