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Flowers were placed near a portrait of murdered Russian ambassador to Turkey, Andrei Karlov, during a Dec. 20 meeting of Russian Foreign Minister Sergei Lavrov with his Turkish counterpart Mevlut Cavusoglu in Moscow. Reuters

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. 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.

Upon shooting Karlov at point blank range at an art gallery Monday evening, Altintas, 22, raised his gun, shouting in Arabic, “We are the ones who swore allegiance to Mohammed for the jihad!”—a phrase commonly used in propaganda materials from the Islamic State, a group formerly tied to al Qaeda, according to Reuters,

The former Ankara riot policeman, who was shot by Turkish security forces minutes later, also urged awareness of the civil war in Syria, where bomb campaigns by Russia, which supports the regime of Bashar Assad, have led to widespread civilian casualties.

“Don’t forget Aleppo, don’t forget Syria,” Altintas yelled, switching to Turkish, according to the newswire. “You will not be able to feel sake for as long as our districts are not safe. Only death can take me from here.”

Altintas also reportedly shouted in Arabic, "Allahu Akbar," or "God is great," which ISIS-linked assailants at the offices of Charlie Hebdo in Paris reportedly yelled as they carried out their attacks on the magazine's staff in January 2015. The phrase, despite an otherwise purely religious meaning, has come to be associated with violence by Islamist groups.

Along with investigating Altintas’ home, police detained his mother, father, sister, roommate and two additional relatives, Reuters reported.