Followers of U.S. Muslim cleric Fethullah Gulen were behind the killing of Russian Ambassador to Turkey Andrei Karlov, the Turkish foreign minister told his U.S. counterpart that Ankara believes, Reuters reported Tuesday.

Turkish Foreign Minister Mevlut Cavusoglu communicated to Secretary of State John Kerry that authorities were focusing their investigation on links between assassin Mert Altintas and the Gulen Movement, a name referring to supporters of Gulen, the self-exiled scholar who was once an ally of Turkish President Recip Tayyip Erdogan before becoming a bitter enemy after a falling out in 1999. Erdogan has previously called for the extradition of Gulen, who lives in Saylorsburg, Pennsylvania, and blamed him for a July coup attempt in which 290 died.

Gulen released a statement Monday condemning Altintas' shooting of Karlov. His statement included criticism of the Turkish government's mishandling of the country's security forces, including massive layoffs and imprisonment. He also urged Ankara to fix the lapses in security that led to the attack.

"No terrorist act can be justified, regardless of its perpetrators and their stated purposes. It is the expectation of the Turkish people and the world that the government investigate the circumstances of this incident, identify those who aided the perpetrator and take the necessary precautions so that such an attack cannot be staged in the future," Gulen wrote.

In response to accusations he masterminded the military faction behind the coup attempt earlier this year, Gulen has denied having any knowledge or part in the event. However, he said he could not vouch personally for all of his followers worldwide. Erdogan has since purged thousands of public officials, accusing them of undermining the Turkish state on Gulen's behalf.

Despite living in self-imposed exile on his 26-acre Poconos compound, Gulen has continued to preach his Sufi-inspired, liberal form of Islam since fleeing Turkey. He has been reaching out to his millions of global followers online through his official website and emphasized studying subjects such as math, science and physics, and peaceful coexistence. He has been called "hocaefendi" or "master teacher" by his followers and is revered for his discipline in prayer, which even includes waking up in the middle of the night to pray.