Jabhat Fatah al-Sham (formerly the al-Nusra Front) has claimed responsibility for Russian Ambassador Andrey Karlov’s murder in Ankara, Sputnik reports.

The group claimed responsibility via a letter published online.

Earlier, Turkish media reported that the number of people detained in the investigation over the assassination of Russian Ambassador to Turkey Andrey Karlov has grown to 13.

The arrests were carried out in the province of Ankara, in addition to the southwestern provinces of Izmir and Aydin, the NTV broadcaster cited police as saying.

Karlov was shot and killed at an art gallery exhibition opening in Ankara on Monday. The Russian Foreign Ministry said it considered the attack a terrorist act.

Previous reports indicated that six relatives of the perpetrator have been detained. Some of them are reportedly linked to the so-called Fethullah Gulen Terrorist Organization (FETO), accused by Ankara of organizing a failed coup in July.