It is a verb here.
In this sentence, ondersoek means investigates / is investigating.
Afrikaans often has words that can function as more than one part of speech depending on context. Ondersoek can also be a noun meaning investigation, but here its position after the subject shows that it is the verb.
Grammatically, polisie is usually treated as a collective noun meaning the police as an institution or force.
So die polisie often works like English the police. It refers to the police collectively, not necessarily to one single officer.
If you want to refer to individual police officers, you would usually use a different expression, such as:
Yes. In a simple statement, the verb is in the second position, and with a basic subject-first sentence that gives you:
But if you move another element to the front, Afrikaans keeps the verb in second position:
Yes. Moord is closely related in meaning to murder.
This is a helpful vocabulary point because Afrikaans and English share many words that are historically related or easy to recognize.
A rough English-friendly guide is:
A few notes:
A more important beginner goal than perfect accent is recognizing that it is one word meaning investigate.
Afrikaans typically uses a double negative with nie ... nie.
So:
That means:
This double nie pattern is a very common feature of Afrikaans.