He's lived in the United States since 1999, when he fled Turkey after being accused of extremist Islamist activities.
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Gülen was once considered Erdoğan's ally. But in 2013 Erdoğan, who was then the prime minister, accused the cleric of being behind a corruption investigation into the government. Erdoğan called it retaliation for shutting down several Hizmet schools.
In the years since, the Turkish government has taken over several companies with links to Gülen, including television stations and at least one newspaper.
The Alliance for Shared Values on Developments, a U.S. nonprofit for Gülen's movement, released a statement after the coup attempt, saying: "We have consistently denounced military interventions in domestic politics. ... We condemn any military intervention in domestic politics of Turkey."
This video includes clips from Fethullah Gülen, BBC, PBS, Samanyolu TV and DHA and images from Getty Images.