Hey there!
Your answer is was not.
This is because the person mentions "I only heard about it afterwards."
This tells us that the person was not there when the crime happened. Also, the other options (besides "was" even though that is not the answer) are not grammatically correct.
Final sentence:
I was not there when the crime happened, I only heard about it afterwards.
Hope this helps you.
Have a great day (:
