I am not sure I understand your argument ; this is you arguing your case to a sceptic ?
Could you please reformulate your statement ?
From my experience, the most effective argument to raise eyebrows is the legal channel : point out the prevailing hypocrisy in regard to judicial proceedings as they relate to terrorist crimes.
Emphasise these events are not solely acts (deeds, or something that has occurred), as portrayed by information media, but criminal acts and admonish your interlocutor to ask himself or herself if he or she considers terror acts crimes.
Your interlocutor will likely agree.
Now that your interlocutor has recognised the true nature of terrorist crimes, turn his or her attention to the topic and role of eyewitnesses in criminal cases, and enquire your interlocutor to offer his or her opinion on whether he or she believes in the necessity of establishing testimonial evidence in criminal cases.
Your interlocutor has now experienced a thought.