Fethullah Gulen speaks to journalists at his home in Saylorsburg, Pennsylvania, Jul. 16, 2016. Reuters

Turkish President Recep Tayyip Erdogan has blamed a group known as the Gulen Movement after Monday’s killing of the Russian ambassador to Turkey, Andrei Karlov. The unofficial organization, categorized as a terrorist group in Turkey, is comprised of people led by a Muslim cleric named Fathullah Gulen who lives in self-imposed exile in Pennsylvania.

Karlov was killed when an off-duty police officer named Mevlut Mert Altintas shot him while he spoke at an art exhibition in Ankara. Altintas yelled “Don’t forget Aleppo, don’t forget Syria!” and shouted “Allahu Akbar!” during the killing.

“The people he lived with before school were detained over FETO,” an unidentified senior Turkish security official told Reuters, using the acronym for the Gulen organization. “It was determined that the people with whom he graduated from school were from an FETO team.”

Gulen’s followers in Turkey are believed to number in the millions, according to the BBC. The movement is known in Turkey as Hizmet, or “service” and is said to be based on a “tolerant Islam which emphasizes altruism, modesty, hard work and education.” Followers of the Muslim cleric are believed to hold powerful positions in the Turkish government. They also run schools around the world, including more than 100 in the United States. Exactly what the organization hopes to accomplish is unclear.

Fethullah Gulen is shown in his home in Saylorsburg, Pennsylvania, Jul. 29, 2016. Reuters

“Hizmet has no formal structure, no visible organization and no official membership, yet it may have grown into the world’s biggest Muslim network,” BBC News noted.

Erdogan and Gulen, formerly friendly, are now bitter rivals on opposing sides of the political spectrum. But a representative for Gulen, Alp Aslandogan, told Reuters there was no connection between the murder and Gulen and called it a “heinous act.”

The Turkish president also blamed Gulen and his followers for orchestrating a failed coup in the country in July. The cleric remains holed up in an estate in Pennsylvania where he suffers from diabetes and heart disease.