Here's some operational and organizational insight of Turkish LE for sleuthers that aren't Turkish nationals:
Turkish LE is highly centralized. Every province is a seperate administrative organization, thus you can hear Istanbul Police, Izmir Police or Ankara Police. However, they are all centrally administered. Operationally, Turkish National Police falls under Ministry of Justice. Administratively and organizationally, they fall under Ministery of Interior. As provincal prosecutors and chiefs of police report to the province governor, which is appointed by the government and works for Ministery of Interior, you can say that MoI is running the show.
SS case is being handled by Istanbul Homicide Branch. Statements of individuals are handled mostly by them. However, it is possible that individuals' statements that are considered peripheral to the case are handled by Istanbul Public Order branch and passed onto Homicide Branch. Public Order branch are the Police you see patrolling streets, initially responding to incidents and mainly manning the precincts.
This Z is almost certainly from underdeveloped Eastern Provinces. He emigrated from his place of birth to Istanbul. Failing that, he must be born into such a family, which previously immigrated to Istanbul.
Eastern Provinces come into play, as Police intelligence and records for those who hail from there are more extensive than they are for from those who are from other regions. Reason for this is simple, there's been a Kurdish insurgency in the region revolving around Marxist terrorist organization PKK for the last 30 years. Easterners almost always face more scrutiny against possible ties to insurgency, their records are more extensively maintained.
This makes identifying Z even more easier. All Homicide had to do is to correspond with Intelligence Branch and Counterinsurgency Branch. Those branches can easily provide information for Turkish Nationals hailing from Eastern Provinces, even more so from what can be fetched from the central databases for any other national not from Eastern Provinces.
Lastly, I assume Homicide is corresponding with Foreigners Branch actively as well, as it is the Foreigners Branch which handles foreigners coming in and out of the country, administrating extended stay and permanent residencies, even manning the border points, checking arrival documentations. Besides, Foreigners Branch must have more English speaking personnel than other branches. I'm sure at least a liasion was appointed to Homicide from them to handle correspondence with US LE.
New generation of academy graduate Police Officers are highly educated though, more than the old school ones, knowledge of English is bound to be more common among them. But again, such Police Officers are junior or middle grade by now, so there's a good chance senior grade homicide people assigned to the case are supplemented by English speaking officers.
I wouldn't be surprised a lot if I learn that some of the assigned homicide guys are watching our correspondence and smiling after seeing theories discussed here.