The boy appeared to be psychologically unstable with a history of threats and attempted violence with knives. He seemed very unhappy, lost, and empty - and it shows in his photographs. From his father's statements, it appears he is unaware of what his son did with his time, where he went, etc., which is why he is speculating. The police aren't bothering with Sulejman's computer, so they don't want to know.
Perhaps the father should investigate and will find clues there.
That's a terrible burden for a father to have. Furthermore, if someone pushed and trained his son to become a murderer, a non-professional could easily erase incriminating files on the computer. (And 95% of fathers can be classified as "non-professionals" when it comes to computers!) What we need is for the SLC police to do their job.