Police discovered al Qaeda-related reading materials at the home of the person who assassinated the Russian ambassador to Turkey in Ankara on Monday, the Istanbul-based Daily Sabah newspaper reported.

Axar.az informs, Turkey police officer Mevlut Mert Altintas shot and killed Andrei Karlov while the ambassador was delivering a speech at an art gallery. Authorities, reporter Ragip Soylu said, citing a “state news wire,” also found books related the Gulen movement, a community of followers of the U.S.-based, self-exiled imam Fethullah Gulen, who Turkish President Recep Tayyip Erdogan blames for an attempted July coup.

The investigation into the killing is mainly focused on links to the Islamist group, officials in Ankara told Al Jazeera. But Gulen, the cleric, quickly condemned the shooting.

"I condemn in the strongest terms this heinous act of terror," Gulen wrote in a statement on his website. "No terrorist act can be justified, regardless of its perpetrators and their stated purposes."

Gulen has, in the past, justified suicide attacks by both al Qaeda and its more violent offshoot, the Islamic State group.