"Η αστυνομία θέλει να ξέρει τη σημασία των συμβόλων."
Μετάφραση:The police want to know the meaning of the symbols.
I think you should also accept "want". Police is a collective noun, both singular and plural verb forms can be used, depending on meaning and usage. Generally we say " the police want to know", ( the same as the Greek) and "the policeman wants to know".