Learn Italian: Il poliziotto ha affermato che aveva arrestato l'assassino. - The policeman stated that he had arrested the murderer.

This sentence uses indirect speech to report what the policeman said in the past. His original words were likely 'I arrested the murderer' (Ho arrestato l'assassino). Because the reporting verb (ha affermato) is in the past, the reported verb takes a step further back in time, changing from the passato prossimo to the trapassato prossimo (the 'had done' tense).

Could aveva arrestato mean someone else had arrested the murderer, not the policeman?

Yes, grammatically it could just mean 'he' or 'she' had arrested the murderer. However, context usually dictates that the subject is the same as the main clause (the policeman). If he were talking about a different person, Italian speakers would typically add a subject pronoun like lui or a specific name to avoid confusion.

What is the difference between using ha affermato and ha detto here?

Affermare means 'to state', 'to declare', or 'to assert'. It sounds more formal and official than dire (to say), making it the perfect choice for a police report or a news broadcast.

If I read this in a novel, would the first verb still be ha affermato?