This is the 13161st most frequent Turkish word.
katilin
The Turkish word "katilin" means "of the murderer" or "the murderer's" in English.
In this sentence, 'katilin' is the possessive/genitive form ('of the murderer') indicating ownership or description.
Katilin kim olduğunu bilmiyoruz.
We do not know who the murderer is.
Here, 'katilin' again means 'the murderer' but specifically describes who performs the described action.
Katilin evde saklandığını söylediler.
They said the murderer was hiding in the house.
In this case, 'katilin' specifies the tracks belonging to or associated with the murderer.